However, these days, with API SM/ILSAC GF-4 certification, "conventional" oils are group 2, group 2+, or mixture of group 1/3, so the performance is much closer to the synthetic levels.

Right, I guess that would define a "quality" "conventional" oil. Can you even buy crappy oil any more? The certs are pretty stringent aren't they?

Synthetic oils are much better suited for extended use for several reasons. Synthetic oils generally have a higher TBN (total base number, a measure of reserve alkalinity) than conventional oils, giving better resistance to acidic contamination from combustion. Synthetic oils generally have better additive packages, allowing the engine to be cleaner and contaminates to be better suspended. Synthetic base oils have a higher VI (viscosity index, a measure of the change in viscosity with respect to temperature), meaning little or no VII (viscosity index improver, the polymers in oil that break down and cause sludge) is needed to give the desired low and high temperature performance. Cold temperature performance was already mentioned, but consider that polyalphaolefin, the base oil in Mobil 1 and Amsoil, is used as a additive in conventional oils to improve the cold temperature characteristics.

 

While conventional oils continue to improve, nothing compares to a group IV synthetic IMHO, at least for now.