I'm sort of interested in switching to a 5/40 syn(but not 100% decided on it) for the summer from the GC, after getting my UOA done on it. Which is the better 5w/40 syn? and for which reasons? 1) Castrol Syntec 5/40 or: 2) Mobil 1 SS Truck/suv (my Walmart carries it now for 4.77 a qt) and where can one find the Syntec 5w/40? the local AZ carries GC, but not this.. BTW: car is 97 T'Bird 4.6L mod, 94k miles, driven daily; 60% hwy, rest city. I MIGHT do extended drains, if the GC uoa is ok. [another"?": Can i use GC for 5k miles or so for the 1st time, and do a UOA, or should I be safe, and get it at 3k miles?] Thanks in advance.