The BMW shop that put in my new used engine (<100K miles) told me to run 15w50 synthetic this winter because it was "an older engine". Why would that be? I know there's a lot of different philosophies out there about oil, but what's the rationale in this case? He said just to warm it up longer when it's real cold out. BTW, the manual says that warming up the engine is not necessary except on the very coldest days. How long do you tend to warm up during the winter? During the other seasons?