It's not just cleanliness, which is from detergent additives. There are also additives to keep the seals properly... ummm sealed.
Well documented facts
may show that synth oil needs additives to swell the seals. Dino oil either does this by nature, or for some reason already had these additives while the early synths did not. Newer seals are designed to not rely on these additives. Most of this info is available on Google, some of which might be reliable

I also would like to add that early Mobile1 was incompatible with Slick50, which i must say i abhore anyway, but still... It Will turn to gel, as warned by the Mobile1 rep at the shop I once worked for (when i had the Geo). Mobile1 also settled a class action suit regarding sludge buildup in the avaiation field.

edit: sorry for undocumented snow-tech. I've been looking this cr@p up for too long already to provide links now.