I haven't tried the Rotella 15-40 yet, been running the 5-40 synthetic since new for about 12K miles. I've been doing 3-5K on OCIs, changing the filter to a new PL14610 at the same time. Probably overkill on the OCIs and especially the filter changes. The oil looks very clean when it comes out, and feels really good still, but I do notice the shifting gets scratchier around 3K or so. That's why I think I might try the 15-40 next summer--from what I read here it has better sheer resistance. The problem is that I usually do a change before the winter lay up, then I ride that oil in the spring until about mid-summer. So I probably won't get to experiment until June or July 2010! With the 5-50 it starts right up, even in the middle of the winter with the temps around freezing. Even though it gets a lot colder than that here, I don't start the cycle unless we get a warm day in the 30s. Usually run it once or twice over the winter up to operating temp for awhile, just to circulate oil and gas.