For what it's worth to this discussion, I do annual changes on almost all my vehicles and equipment, including a Honda Accord V6 (about 6K miles on a mix of Castrol 4 GTX + 1 Syntec), a Ford diesel pickup (again 5-6K miles per annum on 15W40 Rotella-T, just switched to 10W30), and two farm tractors. UOAs (some of them posted on BITOG) confirm no bad results. That said, I agree with the comment above about driving cycles, fuel dilution and such. I live in a rural area, so my vehicles have an almost ideal cycle and do almost no start and stop driving. They get started and drive at least 6 miles (the nearest tiny town) at 50-60 mph before being shut down, and usually more like 20 miles (the nearest larger town). The tractors are usually run for hours at a stretch. With lots of stop and go on my stuff, I'd run six months and UOA.