To put everyone in perspective car is a BMW 2011 335is, inline 6 3 liter twin turbo (N54 engine). I've always used Castrol Edge 0W-40 Euro Forumla (A3/B4) which is a factory recommendation and the only oil with spec I can find here and I suppose the same before I got last year as well was used. My turbos are currently smoking at decel and will need a rebuild. I am chatting with the vendor of a turbo upgraded internal rebuild kit and he told me the following: Running really low oil weight can cause smoking over time, the factory weight is 5w-30 and I always recommend at least 10w30 because the oil weight can thin out in the summer to wear out the turbos. When you run a 0w30 or 0w40 it will it can thin out so much with heat that it will leak past the oil seals. I then ask him: I asked him well doesn't the W viscosity only means when oil is cold and not at operating temperature. He replies I've had people run 0w30 and 0w40 in those cars and they start blowing oil as soon they start when up. When turbos are protected properly they will last over 100,000 miles.I'm running 15w40 rotella in my BMW 335xi all year round. Ok so with those comments, . Once my 0W-40 oil heats up it's going to act just like a 15w-40 or a 10w-40 right? I'm not the one who misunderstands this? Even though, does he have a point that a 0W-40 could potentially leak past the seals more than a heavier cold weight oil? I understand at operating a 0W-40 vs 15w-40 can have slight differences in the actual viscosity and there is a smaller range of viscosity between it's cold and hot but still... I also live in Canada and drive it in winter which is why I also like 0W-40. Considering the turbo job on this car is a good 10 hour what do you guys have to say, don't want to screw my rebuilt turbos when I'll do the job.