Good question. It also depends on the formulation. Mobil One is, I believe a "Full" synthetic. I have been using Castrol Syntec (I've been using Castrol GTX for >25 years) but I recently found out it is not a full synthetic, but is made from by-products from the refining process, so it is based on dino oil. No one really seems to be able to tell me the difference. I guess from looking around that a true synthetic breaks down less that a dino based synthetic, and is more 'slippery'. But that's from websties, nothing official.