How is it thinner?
Consider that synthetic oil is made for engines, wheras petroleum oil isn't made for anything, it's a byproduct. We refine it for our uses, but have to add a lot of things to it to make it keep from ruining our engines.
My understanding is that synthetic oil will actually clean out the engine, which regular oil does the opposite, over time it cruds it up. When a seal gets a crack, the crud makes a seal, and you don't notice the crack. Synthetic oil may clean away that crud, exposing an oil leak.
Synthetic oil protects the seals, but a broken seal is a broken seal.
Synthetic oil has better properties relating to heat and breakdown. Consider that synthetic oil is uniform, whereas petroleum oil is not. The smaller molecules of oil will burn off before the larger ones, leaving them behind, which forms crud. Synthetic oil burns off evenly if at all.
I suggest reading the info at www.amsoil.com. There is a lot of info on oil in general, though of course, they are trying to sell a product. If I were to use a synthetic, after reading some research, I would, however use amsoil. It seems to be a good product, with many years of experience backing it.