I live in texas and I want to know at what temperature is it ok to use 15w-40 weight oil? it gets above 60-70 consistantly in april around here. I am using castrol gtx dino oil after my 5-600 mile run of auto rx and after a thousand or so miles I will use the schaffers but need to know whats best to order, the 10 30 or the 15 40