I use GC oil, but keep two quarts of M1 V twin 20W-50 for the for days when ambient is 85 F or more.
GC probably woud be fine, but I want something a little extra on HOT days. M1 15W-50 would be a good choice for HOT days.
If I lived in Florida where it does not get too cold, I probably would use M1 15W-50 as a all year oil for my genset.