QUESTION: How does a Group III engine oil compare to a Group IV in its cleaning ability?

I have read and understood the potential problems of switching a higher mileage engine from a conventional oil to a synthetic PAO, but is there less inherent risk with switching to a Group III? From my understanding esters act more like a solvent (more aggressive, I assume) and detergents and dispersants more or less attach themselves to sludge and other contaminants and carry it away in that form.

ANSWER: Most engine oils are now formulated with Group II (hydrotreated) or a mixture of Group I (conventional mineral oil) and Group II base oils to meet the latest API gasoline (SM) and diesel (CJ-4) performance designations. Because Group III and Group IV (PAO) base oils are both considered synthetics (since 1999), any oil labelled as a full synthetic, would contain either Group III or PAO, or both.

Any oil labelled as a partial synthetic or semi synthetic or synthetic blend would contain Group I or Group II (mineral oil) plus some amount of Group III or PAO (synthetic). There are no designations for us, as end-users, to know what specific base oils the oil formulator has used, so your question is a bit academic in nature. We would never know if we were purchasing a Group III engine oil vs. a Group IV (PAO) engine oil. But, from an academic point of view, I would expect the cleaning ability of a Group III and a PAO to be similar (generally poor). Again, academically, I would expect less risk of seal issues etc. when switching from a Group III (as opposed to a Group I) to a PAO, as the Group III and PAO are chemically similar. 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 and may cause some seals to swell slightly (they also can remove some paints). These characteristics (not the paint removal) can be beneficial and some oil formulators will add some small amounts of ester base oils into their synthetic (Group III and PAO) formulations to improve these characteristics. - Courtesy of Noria Corporation