:whs:
I've got a sample of 15W-40 Rotella with 2,600 miles on it from my last oil change. I'll send it in for analysis. I'll vary it from there (say, 1,500 to 2,000 miles, then maybe 3 or 4,000 mile intervals) to see if anything changes.
I don't plan on switching oils, so maybe someone else...