Why only change the oil once a year? I suppose one might get by with a 12 month interval at the outside, but I would imagine most people hit the mileage interval well before that. I know some of the Hondas specify a 600 mile interval, some probably longer, but oil breaks down after it has been exposed to combustion by-products. I'd run cheaper oil before I stretched the change intervals that far. Certainly twice a year won't break the bank, and in severe climates, once a month has worked out very well for us (though I do cheat and follow the mileage spec on our Yam/Griz. At 800 miles per change, that means about 5-7 changes per year.)
I say that because I don't use mine year round, i'd be lucky to put 500 miles on in a year. I've had no issues with my motor/transmission/differentials other than swamping it in a high creek a few times. I always check the oil and diff levels before going on a ride and top off as required but usually my fluids are good to go. I know your situation and it is certainly much different than mine.
