So I've been using M1 5w-30 and now just changed to m1 10w30 for the summer months. I've heard a lot of great things about this magical GC that even has it's own sub-forum! Should I change it to GC? I have a Subaru WRX STi that has a turbo-charged 2.5L boxer engine. It sees single digit temps in the winter and 90+ degree in the summer. Here is my recent UOA: 1st UOA: Castrol GTX 5w30 2nd UOA: Castrol Syntec 10w30 3rd UOA: M1 5w30 I would love the input on all the GC fans here! Thanks!