I'm thinking about making the switch from Castrol or Valvoline 5w-30 to a synthetic motor oil. I will not use Pennzoil and I'm not sure if the two brands I like have a synthetic. I hear Mobil has a good synthetic, but I don't know for sure. Anyway what I want to know is do I use the same wait (5w-30) in synthetic or dose that have to change. And what kind of synthetic do y'all use?