Hello, I have a new 07 GMC Duramax which is ready for it's first oil change @ 1000 miles. After spending several hours reading through the archives on this forum, it seems that the prevailing wisdom is to avoid a synthetic for the first couple of oil changes, in favour of a high quality dino such as Delo 400 15w-40. This is clear enough, but what complicates my decision slightly is that I live in a northern climate, and with winter approaching, I would normally switch my vehicles to a lighter weight oil. I know very little about lubricants in general, and virtually nothing about what works best in a diesel. I would appreciate any advice you folks may have. Would the 15w-40 be a good choice in -30c conditions, or should I consider something else?