Oil molecules don't shear. The polymer viscosity index improvers shear. These are tiny bits of plastic that change shape as the temperature of the oil changes. They thicken a hot thin oil so it tests like a hot thick oil. Under mechanical stress these plastic bits get broken (sheared) and no longer thicken the oil. There is a range of quality and cost available in VIIs. The best are extremely shear stable. I looked at a
product data sheet for a better oil than Shell Rotella T synthetic, and even this oil uses "a highly shear stable viscosity index improver." Among the reasons this is a better oil than Rotella syn is that it is about 75% Group III base oil and about 25% Group IV (PAO) base oil. I don't know if a wet clutch shears VIIs. The transmission gears do shear the VIIs.
Shell makes the base oil in Rotella syn by refining crude oil. It has the legal right to be marketed as a full synthetic in the U.S., but is not a "true" synthetic like the polyalphaolefin or the ester base oils. Chevron has great web pages describing their Group III base oil and how it is refined from an intermediate product in crude refining, vacuum gas oil:
http://www.chevron.com/products/sitelets/baseoils/isodewax.aspxAnd here's Chevron's description of the performance of Group III base oil...Chevron calls theirs Unconventional Base Oil, UCBO. Shell calls theirs Extremely High Viscosity Index, XHVI.
http://www.chevron.com/products/sitelets/baseoils/grp3_perform.aspxSo, among so-called synthetic oils there are differences in the cost, performance, and chemistry in the base oils, and differences in the cost and performance in the additive packages including the viscosity index improvers. Probably the best synthetic is PAO mixed with some ester base oil. Group III with some PAO is excellent. The best Group III base oils are very good, and there are some low cost Group III on the market that just meets spec and aren't that great. Shell makes a very good Group III.
Here's a comparison of various base oils showing where PAO is better and where Group III is just as good:
http://www.chevron.com/products/sitelets/baseoils/comp_med.aspxConventional oils will be made from Group II or a base oil above the Group II spec but not quite Group III unofficially called Group II+. A 15W-40 is probably a mixture of Group II and Group II+. A conventional 10W-40 is probably Group II+ with ample VIIs or cheaper Group II with lots of VIIs. Just about every oil, conventional and many synthetics will contain VIIs. Less VIIs and higher cost/quality VIIs are better.
Viscosity index is a numerical index of the viscosity change in oil from 40°C to 100°C. A higher VI number indicates an oil that thins less when hot and thickens less when cold.