The manuals for both of my cars recommend 5W-30. I live in the NE and it can be very cold for 2 months out of the year. So I am considering switching from 5W-30 dino to GC 0W-30 synthetic to run year round in the one car. The other car uses BMW 5W-30 long life. From what I understand the BMW oil is a group III oil, not a true synthetic. My concern is that I've always heard that it is best to try and keep the difference between the 2 numbers as small as possible. Under this theory, a 5W-30 would be "better" than a 0W-30. But many here have said such great things about the GC 0W-30. the W-30 should be a little better in cold winter, but will it be ok in summer? Thanks.