Hopefully, it isn’t a surprise to anyone, but oil is supposed to stay inside your car and not leak out. If it does escape, it’s a pain. An oil leak can cause your car to smoke and makes a mess of your engine compartment, as well as your driveway. The good news is that there’s a variety of products made just to stop leaks. They are fittingly called “stop leak additives.”
- Choosing the Right Way to Stop a Leak
- Top 10 Best Oil Stop Leak Additives 2021
- 1. Best Overall Pick: Lucas Oil Stop Leak
- 2. Honorable Mention: BlueDevil Oil Stop Leak
- 3. Best Premium Pick: Bar’s Leaks OS-1
- 4. Best for Stopping Smoke: Liqui Moly 2021
- 5. Best Budget Pick: STP High Mileage Oil Treatment
- 6. Best Main Seal Repair Additive: Bar’s Leaks Concentrated Rear Main Seal Repair
- 7. Best Fast-Acting Leak Stopper: ATP AT-205
- 8. Best for Head Gaskets: BlueDevil Head Gasket Sealer
- 9. Best Emergency Stop Leak Spray: Permatex Spray Sealant
- 10. Best for Big Leaks: Permatex Ultra Black
- About Stopping Leaks
Choosing the Right Way to Stop a Leak
There are a lot of brands that all make a product meant to stop leaks. Every single one of them claims to be the best. The truth is that there is no universal best. There are some products that work better in some situations, and there are some that work better in other situations. The best way to get the most out of an additive is to choose the one that works best for the problem you are having.
Small Leaks Only
Here’s the thing with stop leak additives: they don’t work on big leaks. They also only work on certain types of materials. Leaks from cracks in metal, leaks from pressed-in fittings, and leaks from loose parts need to be fixed. No amount of liquid gunk you pour into your oil will magically weld a crack shut. If you see a few drops weeping from an old gasket, though, they are perfect.
Where the leak is coming from is probably the most important thing to know when choosing an additive. If you head over to Popular Mechanics, they have a quick guide for finding out what component on your engine is leaking. You don’t actually need to be perfectly accurate, just the general area is fine. A head-gasket leak has a different set of rules than a main seal when it comes to stop-leaks; it doesn’t matter the exact location of the hole.
External leaks are easy to notice. The oil will be on the outside, either coating your engine bay in slime or staining your garage. An internal leak, where oil is getting into the cylinders or cooling system, is much more subtle. Some oil stop-leaks can solve an internal leak, but usually, if you suspect oil is getting into internal components, you are looking at a visit to a mechanic or a few long nights under the hood.
Oil Being Burned
The biggest symptom of an internal oil leak is blue or dark gray smoke coming out of the exhaust. When oil gets into the cylinders, it burns and colors the escaping gases. Unfortunately, while it can sometimes just be a leaky gasket, it’s more likely that there is a mechanical seal that is not working correctly, and you need to open the motor up.
Oil in the Coolant
Oil in the coolant system, and much more noticeably coolant in the oil pan, is a bad head gasket symptom. If there is some cross-contamination and your car is belching out white smoke, stop driving the car. A stop leak will not help you if the coolant is getting into the cylinders.
Gas or Diesel
One final consideration is the type of fuel your car uses. Some additives won’t work with some fuels. Most of the time, additives are formulated for all engines, but there are a few special cases.
Alright, let’s get down to business. Here are our thoughts on 10 of the best stop leaks out there. Most are simple enough to use; just pour them into your oil reservoir at whatever ratio it says. Some are a bit more complicated, but all of them are good products.
Top 10 Best Oil Stop Leak Additives 2021
1. Best Overall Pick: Lucas Oil Stop Leak
The Lucas is great for weeping gaskets. If you have a slow leak on the oil pan or around valve covers, it’s perfect. If it’s a big leak, don’t bother.
Any Compatibility Issues?
Many people seem to believe that Lucas Stop Leak won’t work with diesel engines, but Lucas says there’s no issue. What it may have an issue with is other additives; just make sure it’s the only thing that isn’t oil you put into your oil pan.
Lucas Oil Stop Leak is more than just a gasket fix, and that’s why we call it the best. It quiets the engine down, reconditions oil, and unlike many additives, you don’t have to make sure to drain it out after a while. It’s safe for your engine, and only the BlueDevil stop leak rivals it in terms of total compatibility with petrol-based products. You can use it in any oil, with any engine, and feel ok. It also doesn’t cost much, per ounce, it’s one of the cheapest products on our list. So there’s that.
It’s not a miracle substance, though. For one thing, bigger leaks need filler. Lucas stop leak doesn’t have any filler in it. That makes it super safe, but makes something like the Bar’s Rear Seal stuff better for bigger problems.
There’s also this pervasive theory that you shouldn’t use it on a diesel engine. No one seems to be able to say why, but it does cause some people concern. It’s ok to get the BlueDevil instead if you can’t shake that feeling.
- Improves engine performance in addition to stopping leaks
- Very safe to use
- Not good for bigger leaks
- Doesn’t play well with other additives
2. Honorable Mention: BlueDevil Oil Stop Leak
BlueDevil’s forte is pinhole leaks and tiny cracks. While weeping gaskets are better solved with the Lucas stop leak, BlueDevil is better at stopping a leak with visible drops. It’s a very long-lasting product, so the best leaks to fix are ones you don’t plan on addressing in the future. It’s still not magic, though, so don’t expect it to fix bigger issues.
Any Compatibility Issues?
No, like the Lucas, the BlueDevil contains nothing that can interact with normal engine operation. In fact, it doesn’t have the stigma of being incompatible with diesel motors. If that’s what turned you off from the Lucas, then this is a good alternative.
There’s only one thing that prevents the BlueDevil stop leak from being #1, and that’s the price. It’s virtually identical in function to the Lucas product, but it costs a bit more. That being said, it’s still an excellent product. If you just like the blue bottle better, or don’t like Lucas, it’s going to get the job done, and you’ll be happy.
It does take a bit more time to use than other products if you do it by the book. Which let’s be clear, you should always read and follow instructions when it comes to adding things to your engine. BlueDevil suggests going through the process twice to make sure leaks are taken care of, which means it takes twice as long as others like Bar’s stop leak.
- Works with all engines
- Stops pinhole leaks
- Costs more than similar products
- Takes a little longer to use than similar products
Any time a seal gets old, it has a chance of developing a leak. That’s because it dries out and shrinks a bit. Products like the Lucas get absorbed and work quickly to expand the seal, making it perfect for slow seeping leaks, and the BlueDevil works by expanding over time so that it can close off bigger holes. Bar’s does both, making it perfect for older engines that have a lot of leaking all over the place. It won’t fix a big crack, but it will allow you to finally clean your engine bay.
Any Compatibility Issues?
Bar’s doesn’t sell products that have compatibility issues; it’s something they are known for. You do pay for that peace of mind, though.
If you are not sure what product to get, get this one. It costs more than the others, it comes in an intimidating bottle, and it has funny words printed on the box. Ignore all that. Bar’s Oil Seal has the best chance of working on small leaks. They also sell a product for bigger leaks, the Bar’s Rear Main Sealer, but that’s even more of an intimidating product to use. Stick with this. It’s easy to use, works for older motors, and won’t damage turbos. If it doesn’t work, save up for a replacement gasket.
Just be aware of two things. First, it doesn’t work as fast as the Lucas stop leak. You’ll need to wait for about 200 miles worth of driving time for it to really show good results. Second, it costs four times as much as the Lucas but isn’t really that much better. It’s more of a psychological thing; it costs more, and has really fancy words printed on it, so you’d think it was way higher quality. The truth is that it’s not that much different. We said it was the most likely to work, and we stand by that, but you might kick yourself if you found out later that the cheaper products would have worked just as good.
- Works with any motor
- One of the best working stop leaks on the market
- Is good for older motors
- Costs more than other products
- Takes a while to activate and work
4. Best for Stopping Smoke: Liqui Moly 2021
Liqui Moly is an excellent product for internal leaks. Bad seals on the inside of the motor lead to excess smoke and can cause you to fail inspection. If you see the telltale blue smoke rising from your exhaust, this product can save you a costly visit to a mechanic.
Any Compatibility Issues?
There is one, and it’s a bit of a weird one. Don’t use it on vehicles with a wet clutch that have a shared oil reservoir. If you don’t know what it means to have a wet clutch, you probably don’t have one. It’s rare. Many motorcycles have wet clutches, but the chances of the average driver encountering one are slim. We just have to say it.
Some websites also note that you shouldn’t use it with an automatic transmission: that’s not true. What you shouldn’t do is add it to your transmission. That’s true for most stop leaks unless they specifically say they are for automatic transmissions.
Liqui Moly is German, and it fits right into the Mercedes and BMW precision engineering philosophy. It even looks like it belongs on a shelf at a BMW shop. More than anything else, the Motor Oil Saver is all about keeping the insides of your motor healthy. The Lucas improves motor function as a byproduct. It’s a gasket sealer first, and a stabilizer second. The Liqui Moly is all in on the stabilization and lubrication aspect.
It works fantastic if you are burning oil. It also works fantastic if you seem to be losing oil, but can’t see any leaks. It’s not as good at sealing gaskets, though, so if you see evidence of a leak on the outside of the motor, you are probably better off sticking with BlueDevil or Lucas.
- Great at slowing or stopping engine oil consumption
- Good for making your car run cleaner
- Liqui Moly is a very high-quality brand
- Some compatibility issues
- Not as good at stopping external leaks as other products
5. Best Budget Pick: STP High Mileage Oil Treatment
It will fix really minor leaks. If you have a dribble, get the BlueDevil, and if you have a lot of weeping, get the Lucas. But if you have an old car with mystery stains from oil that seems to have no origin, this stuff is probably all you need. It will help stop smoking too. Not like cigarettes, that’s a different journey, but high mileage engines sometimes leak a little oil into the cylinders. The STP will help manage that.
Any Compatibility Issues?
If you follow the directions, there are no issues. However, and this is important, if you add too much of the product and dilute the oil, you can risk damaging your engine. It’s a very thick product and can make your oil a little slimy if you use too much. That slime will clog channels and make the engine work harder.
STP is a well-known oil product manufacturer, and unlike the band, they are still putting out great products. This high-mileage treatment is one of them. It’s for, as you probably guessed, high mileage motors. It’s also amazingly cheap, easily half the cost of any other additive on this list.
It works well if you use it to stop very slow leaks on older cars. Think of the STP treatment more like an oil thickener than something that will plug a hole. Imagine trying to pour molasses through a colander, and you’ll get it. The downside is that it’s not great for reconditioning seals, especially not compared to Bar’s Stop Leak. Granted, as we said, it’s much cheaper.
It’s also a little bit dangerous if you just use it willy-nilly. Unlike products like the Lucas that don’t dilute the oil, the STP treatment will eventually become concentrated and cause problems if you just keep adding it. Do an oil change more often, and you’ll be fine.
- Good for high mileage engines
- Helps an older car stop smoking
- Can dilute the oil if used improperly
- Not as good at reconditioning seals as other products
6. Best Main Seal Repair Additive: Bar’s Leaks Concentrated Rear Main Seal Repair
The name suggests that it’s only for main seals, but it’s actually suitable for just about any gasket or seal. What it’s best for, and what sets it apart from everything else on the list, is medium-sized leaks. Since it is a relatively thick and heavy-duty concentrate, it can really get into cracks and issues and close them up.
Any Compatibility Issues?
There should not be any compatibility issues. As with the other Bar’s product we have, making it easy to use, worry-free products are tied to their brand identity.
This is the one product to try after other products have failed. It can close larger holes than anything else on this list. There aren’t many downsides to it either; it’s just more hardcore than most people will need.
All stop leaks carry risks. Risks like clogging channels or causing a leak to worsen due to the type of damage a seal has suffered. This concentrate just has a greater risk of all that compared to everything else. It’s more likely to work, and also more likely to cause problems. That’s why we say to try it after something else fails. It’s probably your last chance to solve the problem before having to take off the part and change out the gasket.
- Fixes larger leaks
- Compatible with all engines
- Works quick
- Small risk of clogging oil channels
- Will make improperly installed gaskets leak worse
7. Best Fast-Acting Leak Stopper: ATP AT-205
This is another one of those general-purpose sealers. The “I’m not sure where the leak is coming from, but it’s slow enough not to cause a panic,” type of leak that you think could be a valve cover or could be a worn-out oil cooler line, but you don’t really care to find out which. The only thing that makes it different than the Lucas or similar products is that it’s formulated to be better on plastic parts. Your ‘90s-early 2000s import cars will be happier.
Any Compatibility Issues?
There is one issue to be aware of when looking at the ATP. If there’s gunk in your oil pan already, like some engine sludge or other contaminant, it may dislodge the gunk and clog things up. The chemical that makes it useful for restoring gaskets is also really good at dissolving particles like paint, so just be aware of that. Read the directions and use less than they recommend at first. If it fixes the problem using less, you have no problem and didn’t risk clogging your oil pump.
ATP makes a good product. It’s not as good as the Lucas product, and it costs a little more per treatment. However, if you own a car with many plastic parts, you may find it’s more useful to have the ATP over anything else.
The thing with the ATP is that the chemicals that make it good for polymer seals can also make it bad for your car. If a gasket is rolled or improperly installed, it will do its job and make them swell. That will make the leak worse. It will also strip the paint off your car if you accidentally spill it, so don’t do that.
- Works well on cars with a lot of plastic parts
- Good for minor leaks
- Works faster than similar products
- There’s a chance it will make the leak worse if the gasket is damaged
- Should only be used if you know the oil is clean
8. Best for Head Gaskets: BlueDevil Head Gasket Sealer
There are few phrases that spark fear into the hearts of mechanics. At the top of the list are terms like rod-knock, lifter noise, and blown head gasket. It’s that last one BlueDevil is attempting to solve.
This is a product meant to stop coolant from getting into the cylinders. You don’t pour it into the oil pan. You pour it into your radiator. We chose to include it on a list of oil stop leaks because one, head gaskets can leak oil as well as coolant, and two, these kinds of problems are all connected to each other. Chances are if you have a bad gasket on the bottom end, you have a bad gasket on the top end too.
Any Compatibility Issues?
Tons. We hate to say it, but there are just a ton of compatibility issues anytime you deal with coolant. Since it’s pretty much impossible to know how a product will react with every single one of the hundreds of coolant and radiator types, the best thing to do is to drain and flush the coolant system before attempting to use this product. That’s a tall order, but it’s way better than clogging a tiny coolant vein. The good news is that BlueDevil makes everything you need to do it, and they all work with each other.
The simple truth is that there’s no such thing as a perfect liquid head gasket fix. There are temporary solutions that work fine, and there are products that won’t work at all. Even products like the BlueDevil that work on some vehicles may not work on yours. What you are paying for is the chance that you will only have to buy a product that’s around $50 instead of paying a mechanic several thousand.
The earlier you can catch a problem, the better chance you have. If you notice excess smoke from your exhaust, first try a product like the BlueDevil oil stop leak. If the problem persists past 500 miles or so, then start diagnosing a blown head gasket. Don’t wait with head gaskets, and honestly, don’t put all your faith into a head gasket sealer. It will cost you a lot of money if you let the heads warp because you waited too long.
- Can save you thousands of dollars
- BlueDevil makes a full line of products to help you along the way that are all compatible with each other
- Helps with leaks everywhere on a coolant system
- It might not work for you depending on thousands of hard to predict variables
- It takes a long time to do it right
9. Best Emergency Stop Leak Spray: Permatex Spray Sealant
Any small leak that you can see from the outside. It doesn’t matter what liquid is leaking, and it doesn’t matter what material it’s leaking out of. The Permatex Spray just coats the area in a goo that hardens and creates a barrier.
Any Compatibility Issues?
None. Since you spray it on the outside of the leaking component, it doesn’t actually have any interactions with the inside of the motor.
The Permatex is another product like the RTV that isn’t something you add to the oil, but it is a good thing to have in your tool kit if you are trying to fix a leak. Unlike the RTV, it’s incredibly simple to use. You clean the leaking area, shake the can, and spray the contents at the leak. It hardens into a new seal and dams the liquid inside.
It’s only a very quick fix, though. The hard barrier will be in the way of future repairs, and it will have to be chiseled off, which is a pain in the butt. Basically, use this stuff if you don’t have any other options. Like you are out on the trails and just need to keep it together for another 30 miles until you make it back to civilization or something.
It can be picky sometimes. You have to clean the area pretty well so the goo can stick to the surface. Even then, sometimes, conditions are wrong and it won’t form a good barrier. This just adds to the idea that it’s only a good thing to use if you don’t have any other options and nothing to lose by trying.
- Great for an emergency fix
- Works on a lot of different surfaces
- Easy to use
- Terrible for a long-term solution
- Sometimes it just doesn’t work
10. Best for Big Leaks: Permatex Ultra Black
Unlike everything else on this list, RTV sealer is good at fixing big leaks. That’s because it’s not a product that fixes an old gasket. It’s a product that is used to create a brand-new gasket. Yes, that means it takes a ton of work, but if you are in a situation where you can’t get a new gasket, you don’t have much of a choice when the leaks get too big.
Any Compatibility Issues?
There aren’t really any compatibility issues. Permatex sells other sealers that are better for different applications, but since this article is all about oil leaks, we choose to feature the Ultra Black. The Ultra Black is all about oil resistance, and that’s everything you could want.
Gasket makers are something everyone should have in their toolbox if they work on cars. There’s a lot of stupid reasons you’ll need it. Like if you are replacing a part and accidentally destroy the gasket, or you just need your beater to stop dripping oil on your driveway.
It just takes a lot of work to use it right. Unlike the spray sealer, RTV goop takes a ton of prep and is in no way a quick fix. You have to take the part off, clean it, scrape all old gasket residue off, apply the product, then bolt the part back on before the stuff dries. It’s messy, it’s in no way better than buying the right gasket, and if you have to take the part off again, you’ll be kicking yourself.
- Can fix any oil leak eventually
- Unlike most stop leaks, it’s actually a long-term solution
- Makes a strong, durable seal
- Takes a lot of time to use
- Only a good replacement for a new gasket if you can’t get the new gasket
About Stopping Leaks
Most stop leaks are straightforward, but there are tips and tricks that everyone can use to make sure they get the most out of the product.
Understand the Limitations of Stop Leak Additives
We went over it before, but we’ll do it again. If you experience big leaks, broken components, and cracked lines, none of those can be fixed by adding something to your car. Spray sealers like the Permatex can work temporarily, but nothing will actually fix the problem until you repair the damaged part.
Beware of Diluting Your Oil
Oil additives are not motor oil and do not perform the same function. Even Lucas, who prides themselves on being great for motors, will be bad for your engine in sufficient quantities because it will dilute the oil. As a general rule, only use a product once per oil change. However, like BlueDevil, some products recommend using the product twice if the oil doesn’t stop leaking the first time.
That brings us to our most important piece of advice…
Read the Instructions
Read the instructions and follow them. It’s impossible to overstate the embarrassment you will feel if you bought a head gasket sealer meant to go into your coolant, and you put it into your oil.
Treat All Additives as Temporary
Even the BlueDevil and Liqui Moly that claim to be permanent are not. In truth, even a gasket maker like the RTV is not a permanent solution. You should always plan on eventually getting the correct gasket and installing it.
When a gasket or internal part wears out, it’s only going to get worse over time. A stop leak works by being absorbed into the area around the leak and then expanding. It can only work where it can be absorbed. That means there’s still a lot of the gasket that is not functioning correctly, and over time the same process that caused it to fail in the first place will happen again.
When it happens again, the likelihood of a stop leak working is significantly lower.
Prevention Is the Best Way to Stop Leaks
Keeping your oil clean will keep your internals from wearing down and leaking. That means keeping up on your scheduled maintenance and using the right oils. Heads usually fail due to heat, and lines can deteriorate if the vehicle sits unused. Basically, and bluntly, the most common reason that a car develops a leak is neglect.
We see a lot of questions about stopping oil leaks being asked around the web, so here are some answers.
Are Stop Leak Additives Safe?
Yes. Old additives use chemicals that could cause clogs or corrode materials, but modern stop leaks are safe. You can read all about what modern stop leaks are made out of here if you’d like. In short, assuming you follow the directions, your car will be safe if you use these products.
Can You Use Oil Stop Leaks for Transmissions?
Sometimes you can. Some additives are perfectly safe for transmissions, like the AT-205 Re-Seal. Some are not, like the Liqui Moly. If you have a leaking transmission, though, it’s probably better to buy a product specially made for transmissions.
Can Different Stop Leaks Be Used Together?
In general, no. While it may not cause a problem immediately, it may dilute the oil and cause future problems. It is also not worth it; most of the time if a product doesn’t work, it’s because the leaking part has failed. It’s not because a different product would be more effective.
How Long Should You Wait Before Deciding a Stop Leak Didn’t Work?
If a product is going to work, it will work after driving 500 miles or so. It does vary product to product. For instance, the BlueDevil additive takes longer to activate than the ATP, but both should be at full strength by the 500-mile mark. If you still have a leak after 500 miles, you probably need to look for a new solution.
Stopping Leaks Keeps the Stains Away
No one likes to try to clean oil out of concrete, and no one likes to scrub oil out of an engine compartment. For the sake of sanity, grab some oil stop leak additive and keep the leaks at bay. You should also consider picking up an oil drain pan to protect your garage floor or driveway while you sort out your leak problems. Also, consider using laundry detergent to scrub oil stains out. It works great.