How Bad Is Synthetic Oil For Older Cars – The Truth


There is a myth going on the internet that synthetic oil is bad for older cars. Not sure who started the rumors, but I will do my best to end them. Synthetic oil is actually not bad for older cars and older engines. The synthetic oil can only prolong the life of an old engine that has wear and tear. Read on as I go into detail about why synthetic oil is not bad for older cars.

Can You Use Synthetic Oils In Older Cars and Older Engines

How Bad Is Synthetic Oil For Older Cars - The Truth

You can use synthetic oils in older cars and older engines without any issues. It doesn’t matter if your car is 30 years old or a classic, there is often a synthetic engine oil available for it. The idea that synthetic oils are bad for older cars and oiler engines comes from bad experiences with the early synthetic motor oils.

The synthetic oils are nothing new. They have been around since the 1970s and have been widely used. The idea that synthetic oils are bad for older vehicles comes from bad experiences in the past. Some of the early synthetic motor oils were ester-based and some engine seals are also ester-based.

The engine seals and synthetic motor oil being ester-based can cause swelling of the engine seals and if the swelling is small, it can actually be a good thing. But, if the ester concentration of the synthetic oil is too high, you could see excessive swelling of the seals and a drop in the seal’s hardness and strength.

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It is important that all of the engines seals to maintain their properties regardless if the engine is hot or cold. The last thing you want from your engine seals is to get brittle or lose strength. The thing with the early synthetic motor oils with base stock formulations was not compatible with the engine seals. So, people often found oil leaking all over their driveway.

So, how do we know that modern synthetic oils won’t damage the engine seals? Well, there’s all kinds of testing to ensure that doesn’t happen which is why it’s no longer a problem. For example, a Mobil 1 synthetic motor oil was commercially introduced to the United States in 1974.

Obviously, at the time, there were far fewer industry requirements today if you look at the back of a bottle of oil you’ll see API certification, ILSAC certification, and various manufacturer certifications. These certifications have extensive testing and they’ll include seal compatibility tests to ensure that what’s inside the bottle is actually compatible with the seals used in engines.

Can Synthetic Oils Cause Leaks In Old Cars

Synthetic oils can cause leaks in old cars. The synthetic engine oil has a higher concentration of dispersants, detergents, and other cleaning elements which can remove any sludge or gunk buildup that could be acting as a seal and cause a leak in old cars.

Synthetic oils have more cleaning power and will remove built-up sludge and gunk within your engine exposing leaks. But, cleaning your engine is never a bad thing. Keeping an engine clean is never a bad thing especially if you have sludge or gunk building up. That sludge can plug up oil drain holes which lead to more sludge forming on the top end of your engine which then makes your engine hot and can lead to overheating.

If your engine leaks oil, that is an issue that has to be addressed as soon as possible. There’s probably a failed seal. But, sludge and gunk building up around the seal that failed is not a solution. It’s just another problem. Using sludge as the solution would mean causing your engine to fail sooner so that it doesn’t leak oil.

Should You Switch To Thicker Oil If Your Car Is Consuming Oil

You should switch to thicker oil if your older car is consuming oil. The general rule is to always use the viscosity your car manufacturer recommended. But, as we all know, engines don’t last forever. As your engine gets older, the clearances within the engine change due to wear and tear, and your car could start to consume oil. Once your older car starts to consume oil, you should switch to a thicker oil.

Let’s say you have an old car and there’s quite a lot of miles on it. While you’re sitting idle you have relatively low oil pressure and the oil pressure light comes on. The oil that you have been using so far may have been fine for maintaining oil pressure. But, as the clearance within the engine increases due to wear and tear there is more space and the oil might be too thin to maintain pressure in the wider areas. So, using a thicker oil could prolong the life of the engine.

The important thing here is understanding that you shouldn’t proactively switch to a thicker grade oil without any signs that your engine is old, worn and burning oil. Bumping up an oil grade should be thought of as a last-ditch effort of prolonging the life of an engine that’s clearly on its way out.

The thicker oil could help, but it’s not going to change the fact that your engine has significant wear. Higher viscosity oils also tend to have lower volatility which means less of the oil burns off over time. As far as an engine that’s leaking oil, changing to a thicker oil isn’t addressing the fact that the engine seals have failed and are leaking oil.

When Should You Use a High Mileage Oil?

Once your engine starts to wear and is burning excessive oil or leaking oil, you should switch to high mileage engine oil. The high mileage engine oil has a seal swelling agent which serves the purpose of reconditioning engine seals as they harden over time.

Over time, the engine seal can harden, shrink, and lose its elasticity. The swelling agent in the high mileage oil is designed to recondition that seal so that less oil makes its way past. Now, this doesn’t mean the seal is permanently fixed. If your engine’s issue is a broken seal, then the ultimate fix is replacing that seal. But, a high mileage oil can help prolong the seal’s useful life if it has started to deteriorate. You can try and swell an old seal back to its original state. But, obviously, you wouldn’t want to proactively try to swell a brand new seal.

If for some reason you were to start using a high mileage oil early, it’s not going to harm your engine as long as you can see it meets all of the required industry tests.

A common spot where a failed seal can cause you to burn a lot of engine oil is the valve stem seals. These seals prevent oil from within the cylinder head from entering your intake manifold. If the seal deteriorates or fails, the oil will drip down onto the intake valves. Then it will move into the combustion chamber along with the intake air and burn up in the engine.

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