I'm seeing a fairly even split in failures, internal vs. external. The first few that I saw (years ago) were generally loose wheel splines. Haven't seen one of them in a while, though. When they went away, all that was left was worn pinion bearings. Sometimes this would heavily contaminate the oil. Then the big bearing failures started. Currently I have three in the shop: one with a bad (dry) big bearing, one that had a bad big bearing and is back because the new bearing spun on the axle F800-style, and one I think is a worn pinion. My personal opinion is that the pinion bearings wear due to improper setup. Oil changes may prolong the deterioration, but won't stop it. And of course the big bearing failures can't be traced to the oil in any way that I can see. The big bearing is pretty well protected from direct spray, but it'll get wet. I don't dismiss Ron's comments at all.