Drag racers around here tell me that the only difference in the 5W-40 and 15W-40 Rotella oils in cold weather (forgetting all other differences for the moment) can only be noticed below 0 degrees. I don't ride my 525 at zero. 30~32 and up is more like it for me. They claim the 15W-40 flows the same at 32 as the 5W-40, contrary to the published specs from Shell. Perhaps there are other variables that are throwing them off, such as zinc content.
Stu