I do not switch to synthetic til the engine is broken in. I have been using Amsoil and one year had to do a valve job on a Subaru engine with over 100,000 miles. There was no ridge in the cylinders and the hone marks from the origionaol build were still faintly visible.
I switch to all shynthetic oil, engine, transmission, differentials and in this kind of weather, single digit and below zero it is worth a couple of mpg because of a lower pour point. It also has higher tolerance for heat before coking. Subaru #11 went to the junk yard with over 300,000 miles and these are a high RPM engine.