I'd like some opinions on optimal oil change intervals on various engines that don't even come close to hitting the manufacturers recommended mileage/hour change intervals within a one year period. (ex: diesel tractor, car, and outdoor power equipment) Here are some scenarios: My Supra is driven about 2500 miles per year and I use Redline 10W30. Can I safely let it go to 5000 miles (ie 2yrs)? The miles it does get driven are not short trips. It gets a full workout when driven :-). I have an older BB Trans Am that only get's driven a couple hundred miles a year. If I change an expensive synthetic oil annually it seems like a total waiste. (like pouring brand new oil down the drain). I also have a 7 yr old John Deere diesel tractor that has yet to hit JD's official oil change hour threshold. (but I do change it every 2 yrs anyway) I use premium synthetic in everything and except for the Supra, I usually never go more than 2 yrs. (Supra never goes more than 1 yr). Is 2 yrs too long for infrequently used engines? Would it be better to use "el cheapo" dino oil and do a change annually or use a premium synthetic and keep the change to about 2+ yrs? Based on a previous post I think I'll use Amsoil 15W40 AME (Diesel and Marine oil) in both the Trans Am and the diesel tractor. I don't think it would be appropriate for the Supra though. TIA, this is a very informative group.