I remember my (scientist/phd/Dr.) friend for Castrol in New Jersey (visited his lab that he was developing new cleaner fuels) tell me that multi weights add polymers to improve viscosity, but they are not oil. But since temperature changes are drastic we can't change weights all the time. But straight weights are all oil, no polymers. I run 5 20 synthetic in everything (from a supercharged mustang, Lexus, to old cub cadets, lawn mowers, sawmill engine, etc...lots of engines. Diesel ram and tractors get Rotella synthetic)
5 20 synthetic even in my old apex and vectors, and rx1's, but once in a while put 0 40 synthetic in. Right or wrong, who knows. Never had any issues. 2 stroke chainsaws and weedeaters get only Bombardier injection oil at 50:1. Even my 1970's era 700 pro mac Mccullochs get it and love it. The relatively clean synthetic (from the lexus and mustang) used oil is my bar oil. I have very little bar wear (the burr you get near the chain slot over time) using synthetic compared to regular bar oil.