Every since I first got my BB512i in Dec 2002 I have been using Castrol Syntec 5w50(winter) or Castrol TRW 10w60(summer) and my car never smoked. About a month ago Sams had Mobile1 15w50 on sale so I bought two cases and used it to change the oil/filter in my Boxer. As soon as I started the engine in the garage I noticed smoke coming out of the tail pipes; not a whole lot mind you, but since it didn't smoke at all before it was enough that I noticed it immediately. Thinking that maybe I just overfilled it a little (although the level on the dip stick appeared to be correct) I drove it a couple of weeks yet it continued to smoke. So, on Saturday I again changed the oil/filter this time back to Castro 5w50...and guess what ?...no more smoke. Why do you think that my car would smoke with Mobile1 15w50 but not with Castrol Syntec 5w50? Any oil experts out there have an opinion. In any event NO MORE Mobile1 for me!