I have a new to me 2011 VW Tiguan SEL 4Motion with 18,000 miles with the 2.0 Turbo gas engine. It has Castrol 5w-30 full synthetic in it which is what the previous VW dealer uses (I know this because I called them). The dealer I will be using in the future puts in 5w-40 Castrol Professional OE Full Synthetic (I realize that could well be Group III and not a "true full synthetic"). How would one rationally choose between a 502 certified 5w-30 v. a 5w-40 v. a 0w-40? Is the viscosity more important or would brand be more important? In other other words, would a brand other than Castrol be clearly better? Climate-wise, I live in Lenexa, Kansas. Typically, it gets into the 100's during the summer but so far this year has been unusually cool. It hasn't gotten really cold during the last couple of winters. I am used to Japanese cars and both 0w-40 and 5w-40 seem awfully thick from an initial flow point of view. All opinions will be appreciated.