Older motors, like my 70’s dirt bikes, were designed for the lubricants of the time. Hence, the 20:1 recommendation. Even the cheapest Walmart premix now is far superior to what was available then.
But, by all means, if you know in advance that a heavy load scenario is likely, especially with high operating temperatures —say like a dirt bike tractoring through deep sand on a hot day — go by the manufacturer’s recommendations or reasonably close to it (i.e. don’t use 80:1 if the owner’s manual calls for 40:1).
I don’t have a gas chain saw but being air-cooled and used in high-stress conditions I imagine the use can fit into the “severe” category quite easily.