The transmissions in my truck call for SAE-50 engine oil. Only 50wt is Valvoline racing @ $3.XX A QT and I need several gallons. I was thinking that SAE40 would be ok. It's a little more common here. Pep boys is sellin Exxon and Kragen has Shell on sale in SAE30 but not 40. 30 probly too thin? What about 20w-50? Ideas?