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Switching from Conventional to Synthetic Engine Oil


Synthetic oil chemicals being analyzed using laboratory beakers and test tubes

Synthetic motor oil undoubtedly provides many benefits over conventional motor oil (or "dino" oil, as many people refer to conventional, because petroleum is a product of fossils of dinosaurs and other ancient living things).

Modern synthetic motor oils provide better wear protection, help reduce sludge formation, offgas fewer vapors that can foul the intake valves in GDI engines, and have better viscosity and temperature stability. All those benefits make switching to synthetic oil a no-brainer, right?

Unfortunately, that's not as easy a question to answer as some people believe. The benefits of switching to synthetic oil have to be balanced against the risks. Those risks increase with the age of the car and the time it has been using conventional oil. They're also affected by how often the conventional oil was changed and the quality of the oil that has been used. A poor (or unknown) maintenance history increases the risks associated with switching to synthetic oil, as does a history of using cheap, low-quality oil.


What Are the Risks of Switching to Synthetic Motor Oil?

Ironically, most of the risks of switching to synthetic oil have to do with its superior ability to not only reduce sludge formation, but to remove sludge that's already in the engine. Both the synthetic base oil itself and the superior detergents used in high-end synthetic oils can dislodge considerable amounts of sludge from an older engine.

Engine oil sludge is basically gelatinized oil, usually mixed with various other physical and chemical contaminants. As a general rule, it's a bad thing. It clogs oil passageways, reduces lubrication efficiency, and can serve as a reservoir of corrosive chemicals. If you have a new car, doing everything you can to avoid sludge formation (especially using high-quality oil) almost certainly will prolong the life of your vehicle's engine.

This is also true for the engines of older cars, but with an important caveat.

The Synthetic Oil Sludge Dilemma in Older Car Engines

Sludge in an older car's engine is just as bad as sludge in a newer car's engine. The problem is that sludge may also be serving as a sealant in an older car's engine, especially one with a poor maintenance history. As any car ages, seals and gaskets begin to wear; and in an older car's engine, accumulated sludge may actually be acting as a sealant to keep the oil inside the engine.

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That's why you may have heard people say that synthetic oil causes older engines to leak oil. It really doesn't. What is does very often do, however, is expose existing leaks by washing away the sludge that had been acting as a de facto sealant. In other words, the leaks were already there. The seals and gaskets were already worn. But the sludge was plugging the leaks

Is that a good thing? No. We really don't want to be using sludge as a sealant. Removing the sludge and replacing the seals would be a much better approach. But it can also be expensive, especially if the engine's main seals are involved. That particular job is beyond the scope of what most shade tree mechanics can do in their driveways.

Another less-expensive thing to try if your car starts leaking oil after switching to synthetic is to switch to a high-mileage synthetic motor oil. High-mileage oils have additives designed to swell and rejuvenate engine seals, which very often solves minor oil leaking problems. High-mileage oils also contain stronger detergent packages to help remove carbon deposits around the piston rings that can contribute to oil burning.

I recommend that you try high-mileage oil only if you actually have oil leaks or are burning oil in a car that has 75,000 miles (about 120,000 km) or more behind her. Swelling a seal that's not already leaking may cause it to wear out sooner. Besides, if it's a newer car, you probably can get leaking seals or oil burning problems repaired under the warranty.

What it comes down to is that there is a chance -- by no means a certainty, but a chance -- that switching to synthetic oil in an older car's engines will expose leaks that had previously been sealed with sludge. The newly-exposed leaks may be easy to fix with a high-mileage motor oil, or they may require expensive replacement of seals and gaskets.

Are the Benefits of Switching to Synthetic Oil Worth the Risks?

That depends a lot on your plans for the car's future. If it's a classic car or a family heirloom that you want to keep running forever, then presumably you are willing to do everything you can to make that happen -- including changing the gaskets and seals if they start to leak. In that case, by all means consider switching to synthetic.

If you're a 17-year-old with a rusty old beater that you just need to keep running until your parents buy you that newer car they promised you when you graduate high school, on the other hand, then maybe you just want to leave well-enough alone.

My Recommended Way to Switch to Synthetic Motor Oil

If you've made up your mind to switch from conventional motor oil to synthetic, I suggest that you use a sludge-removing engine flush between draining the dino oil out of the engine and replacing it with synthetic. The one I like best is Liqui Moly 2037 Pro-Line Engine Flush; but Lubegard 95030 Engine Flush or Sea Foam Motor Treatment are also good choices.

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I should mention here that as a general rule, I'm not big on engine flushes. Sometimes they can do more harm than good by partially breaking down sludge in areas where it's doing little harm, and and sending it on its way to clog smaller oil passages in the engine. That's not the norm, but it does happen sometimes, especially in cars that have been very poorly maintained.

The reason I recommend flushing the engine before switching to synthetic oil is that synthetic oil itself breaks down a lot of sludge; so it makes sense to remove at least some, and hopefully most, of the existing sludge while the old oil and oil filter are still installed. That will help reduce the chances of the synthetic oil dislodging enough sludge to clog the new oil filter and cause oil starvation.

Use the flushing product of your choice according to its label directions. Once that process is complete, drain the oil as completely as possible, replace the oil filter with one made by your car's manufacturer or a quality aftermarket filter like those made by Wix, and refill the oil using the proper amount of the synthetic motor oil that you've chosen.

If your engine was already leaking oil before switching to synthetic (or if you were using a high-mileage conventional oil), then I suggest that you use a high-mileage synthetic motor oil when switching to synthetic. Otherwise, I suggest that you not use a high-mileage oil until and unless your car's engine starts leaking oil.

Keep the first oil-change interval after switching to synthetic very short. I suggest no more than about 2,000 miles (about 3,200 kilometers) if you did the engine flush, or about 1,000 miles (about 1,600 kilometers) if you didn't. This is to reduce the chances of dislodged sludge clogging the oil filter. Keep the second oil-change interval after switching to synthetic short, as well. I suggest no longer than about 3,000 miles (about 4,800 kilometers). After that, follow the car manufacturer's specifications.

You also should monitor your car closely for oil leaks after switching from conventional to synthetic motor oil. If you find leaks, either replace the leaking seals or gaskets, or try using a high-mileage synthetic motor oil for the next oil change. They work often enough to be worth a try.

Be sure to follow good safety practices whenever working under a vehicle. You can find generic instructions for doing an oil change here.

Can You Switch Back to Conventional Oil After Using Synthetic?

Assuming that your car's manufacturer doesn't require synthetic oil, then yes, you absolutely can switch back to conventional if you want. The thing is, there's usually no point in doing so.

Some people want to switch back to conventional oil because their cars started leaking oil after switching to synthetic. As I mentioned earlier, that happens when the synthetic oil washes away the sludge and gunk that were acting as sealants. Synthetic oil doesn't cause leaks. It merely reveals leaks.

This also means that switching back to conventional won't stop the leaks. The sludge and gunk that were acting as sealants have already been washed away. Switching to conventional isn't going to put the sludge and gunk back -- at least not for a very long time -- nor would you want it to. If your car's engine begins to leak oil after switching to synthetic, then either replace the leaky seals or try a high-mileage motor oil like Castrol Edge High-Mileage Oil.

Both conventional and synthetic high-mileage oils have similar seal-conditioning additives, by the way. As far as addressing leaks is concerned, neither is better than the other. So once you've made the leap to synthetic oil, there's rarely a good reason to go back to conventional.

Motor oil being poured from a jug. Laboratory flasks representing synthetic motor oil. A drain pan being used to catch used oil being drained from a car engine. The open top of an engine coolant reservoir. Mechanic using a rooling creeper to work under a car. A hydraulic floor jack being used to raise a car. Three automobile engine oil filters. A mechanic using a wrench to work on a car engine.

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