Will changing to synthetic engine oil damage the seals?

Owners of recently purchased pre-owned cars often want to start off with a fresh charge of synthetic motor oil, but are worried that synthetic engine oil damages seals.

Many people believe that switching a higher mileage engine from a conventional oil to a Group IV synthetic polyalphaolefin (PAO) can lead to seal leakage: but is there less inherent risk with switching to a Group III oil? The rational here
is that, esters are more aggressive detergents.

Before we answer this we need to understand some of the differences between synthetic oils.

Most modern engine oils are now formulated with Group II (hydrotreated) or a mixture of Group I (conventional mineral oil) and Group II base oils to conform to the latest API gasoline and diesel performance requirements.

Since 1999, Group III and Group IV (PAO) base oils are both considered synthetics, and as such, any oil labelled as a full synthetic would contain either Group III or PAO, or both.

Any oil that is labeled as a partial synthetic, semi-synthetic or synthetic blend would contain Group I or Group II (mineral oil) plus some amount of Group III oil or PAO (synthetic). Keep in mind that there is no minimum amount of synthetic base oil required in order to call a blend a semi-synthetic lubricant.

There are no designations for end users to know what specific base oils the oil formulator has used, so we would never be able to tell if we were purchasing a Group III engine oil vs. a Group IV (PAO) full synthetic engine oil.

However, the cleaning ability of a Group III and a PAO would in most cases be similar. Therefore there should be less risk of seal issues, etc., when switching from a Group III engine oil (as opposed to a Group I ) to a PAO, as the Group III and PAO are chemically similar. So it’s highly unlikely this change of synthetic engine oil would damage seals.

So does this mean that a change to synthetic engine oil won’t damage seals?

Not quite: Ester synthetics have a higher degree of solvency than Group II, III or PAO base oils. This means they will dissolve additives and deposits more readily which could cause some seals to swell slightly. They can also remove some paints.



These characteristics (other than paint removal) can be beneficial, so some oil formulators will add small amounts of ester base oils into their synthetic (Group III and PAO) formulations to improve these properties. It’s in this formulation where some older vehicles could experience problems.

It’s important to understand that changing to a synthetic engine lubricant containing Ester does not mean that synthetic engine oil will damage seals, this is a myth – it’s more a factor of the condition of the seals prior to changing lubricants.

If you’re looking for a quality synthetic engine oil that is right for your car, give Habot Synthetic Lubricants a call for a synthetic engine oil that doesn’t damage seals.