EDIT: Yes, every BMW enthusiast forum, unofficialBMW.com, people that love the "old German cars especially BMW and Mercedes...." .. will tell you 20W-50. They are not wrong..
They ARE wrong especially for L6 and V8 Bimmers..
90% of those cars you see have 20W-50 in them.. They are not wrong. Just not "optimal" perhaps. As i said, it is up for debate, the specifics.
Yes, especially in Florida GC 0W-30 would be a fine choice.
Another heavy 30wt option would be G-Oil 5W-30.
btw, the message boards are the prime advocators of "20W-50" with one or two members recommending 10W-40 due to the principles of motor oil and engine lubrication that you talk about, CATERHAM. This wasnt a dig at you. Just talking about the car, ok?
The message boards are PETRIFIED of oil thinner than q0W-40 for the most part, but you will always see one or two posters that explain why they dont use uberthick 20W-50 and the other posters always say "Yes, you are bold." They have never been here. Guarantee it.