I'm a farmer and constantly send in oil samples for analysis. When you pay $200,000 plus for a machine oil analysis is cheap. Anyway this is what I know. Synthetic will out last conventional by at least 250 hrs. I change at 100-150 hrs. per tractor. By consistently monitoring my oil through analysis when I first tried synthetic--- lab says I can change between 400-750 hrs. In a vehicle this translated to 7500 miles. Ok great but it drives me nuts I just simply can't break the 2000-2500 mile limit that I impose because I run is such dusty conditions. What else---The biggest thing in my view is on synthetic your; motor, gear case, etc. will typically run minimum 10 deg. and max 20 deg. cooler. I came down to the rule if I plan on keeping this forever--synthetic. The only down side is synthetic will definetly find a weak seal fast and leak like a sieve. My old three lunger tractor does not get synthetic for this reason, but it's a runner not a fixer or by any means tight. Hope that helps.
