Synthetic oil is not "thinner" than mineral oil. For multi-viscosity grades, the lower number represents a range of absolute viscosity at 32F and the higher number represents a range of absolute viscosity at 212F. These viscosity ranges for a given "weight" are defined by the SAE. For a mineral-based oil to achieve a 20W-50 rating, a "viscosity index improver" has to be added, and these additives are slowly consummed as the oil ages. Straight mineral base stocks cannot achieve a multiviscosity rating without VI improvers.

A synthetic base can achieve a wider viscosity range of 15W-50 with little or no VI improver , so it will maintain its viscosity index for a longer use period. If anything, a 20W-50 might be a 20W-40 after it's used, but the thinning due to loss of the VI improvers is offset to one degree or another by thickening due to oxidation.

There are two basic advantages of synthetic - greater oxidation resistance, which means they can be used for extended drain intervals, and wider viscosity range, which is an advantage if you want to use a "year round" oil such as 0W-40 in below zero winter temps and 100+F summer temps. That's it!

Beyond these two differences, the two behave the same inside the engine.

Duke