Well he's not "right" as in tcw3 won't and should never be used in a chainsaw. That's been proven. It may not be what the manufacturers want, but it works fine.Windthrown is right.. and syn oils are no harder on rubber parts IME. I would worry about ethanol much more in that regard.
As far as the ethanol, we have only had e free that I've noticed for 2 years here. The 51, my 625, and concrete saw have only ever ran on pump 87 and tcw3. Original rings were worn on the 625 (1988 model) and the concrete saw (1996). As I stated, the ts400 needed a new fuel line.
So maybe it's the tcw3/pump 87 combination that doesn't eat rubber. Idk. Everything on the 51 is like new inside.