Is synthetic engine oil bad for older cars?

Besides the viscosity rating of engine oil, we also have the choice of what type of oil we’d like to use in our car – mineral oil, semi-synthetic oil, and fully-synthetic oil. While most of us would agree that fully synthetic engine oils are the better choice, some older cars show problems when fully-synthetic engine oil is used.

We’ve talked about their differences before, and the main advantages of fully-synthetic oil would be the more consistent molecule size and the longer service interval. While some synthetic oils are ester-based, synthetic oils mainly come from crude oil, just like mineral oil. They are just further refined.

The main problem exhibited by older cars with fully-synthetic oil is oil leaks.

So, this whole argument of older cars and synthetic oils is mainly concerning leaks. And the topic of leaks mainly concerns the seals of your engine.

Q: What causes the leak?

A: Some say that the “smoother” texture of the synthetic oil causes it to seep through the seals. The truth is, the chemical content of fully synthetic oil may sometimes not be compatible with the seals in your engine.

Q: What do you mean not compatible?

A: New synthetic engine oils contain additives (e.g. detergents and swelling additives). Some fully-synthetic engine oils are also ester-based. While these additives (or oil base), work well with modern or high-performance cars, they are not always great for older engines with older seal materials.

Check this table out to get an idea of compatibility:

Engine seals have to function well under high-temperature and high-pressure conditions, so you can’t ignore them.

When not compatible, the older seals may become too soft or swell too much when in contact with these additives/oil-base. There’s nothing wrong with swelling additives. They are designed to create a stronger seal. There’s just an issue of compatibility with older seal materials.

Some engine oils even have a seal of approval from car manufacturers so you can use that to help you.

Q: Is there anything else?

A: Detergent additives in fully-synthetic oils may also remove dirt that is plugging a leak near the seal. So, what fully-synthetic oil does is simply expose existing leaks in the system. What you need to do is replace the seal.

Detergent additives, as the name suggests, clean the engine. While dirt/gunk can function as a temporary seal , letting it continually build up will only result in a shorter lifespan of the engine.

Q: Is fully-synthetic oil bad then?

A: Of course not. It’s fantastic (Rotary engine disagrees). Synthetic engine oils have a more consistent molecule size and last longer.

So, what oil should I use for my old car?

If it is a beater car, your best bet is to go with mineral or semi-synthetic oil (why do you want to spend so much on a beater car anyway?). If your doubtful of the compatibility of synthetic engine oils, you can just go mineral or semi-synthetic. 

Just be sure to pay attention to any oil leaks (and low oil pressure warnings). This is part of the old-car-ownership experience.

If it is a classic car restoration, I’m pretty sure you’re going to spend good money on an engine overhaul anyway, so get new seals (don’t cheap out here) and get a compatible engine oil to with it (it could be synthetic, no problem).

Whether it is mineral or synthetic, remember that the keyword is compatibility. Some oil bases and oil additives are just not compatible with some oil seal materials. And as boring as this sounds, the owner manual is your best reference.