Car has run well for 2 decades on Castrol 20w-50, Mobil 1 15W-50, and Redline 10W-40. Do I dare to try GC 0W-30 in this engine during the summer? I have 100 quarts of GC so it is tempting to use it. I feel that it should work during the winter, but I'm not sure about summer use. Also, any opinions about Mobil 1 Turbo Diesel 5W-40 or Shell Rotella T 5W-40 as an alternative to Redline 10W-40? I am very happy with Redline but sometimes I need to do an oil change on the road and don't have accesss to it.