really, its personal preference. lol, i was running castrol enduron 15w40 semi syn all year, now i switched to syntec 5w40 for this winter, liking it very much for the starting, think i may run that all year, any 40 weight oil flows the same once its hot, so if u want to run 0w40 in the summer, its the same as 15w40 once its hot, its just flows better at lower temps.