BMW manual has oil viscosity chart that shows 5W30 oil being used up to 60 degrees. Above that temp. it looks like a 40 wt oil is needed. Why are so many people running 30 wt. oils that live in areas that get above 60 degrees. Does it have anything to do with the use of synthetic oil? Would I be better off using a 5W40 over a 5W30 oil in new york? I wish I knew more about oil.