Oil wieght for turbo motors

[Caliturbo88]

Alright. I just got an 88 turbo supra and the whole motor was rebuilt with new forged pistons and freshly machine block and head so everthing is "tight". The guy we bought it from is a master toyota tech and he runs 20w50 oil in it right now. I was wondering if this thick of oil is really nessicary for winter time. I was thinking somthing more like straight 40w. Also, do synthetic oils work fine for these turbo motors? What brand oil does everyone like? Im a firm believer in castrol oil for toyotas. I searched but couldnt really find anything, so dont nag on me too much.