Tuning the carb is the same as any other engine. You might get detonation if you go too lean, but unlikely in a .40 or smaller. Gallons of fuel? Just whatever it uses? Life, pretty long. Though, I think you would see less life in a car. Maintenance: adjust valves after break-in and occasionally after that, do a search in this forum, it comes up almost daily, and after-run oil in the crankcase for an airplane version. Engine temperature same as a two stroke, 200-280°F at the head. Rebuild: New ring and liner for an OS, new ring and cylinder jug for a Saito. All current production four strokes are ringed, except the OS FL-70. ASP supposedly made ABC four strokes, but I have never seen one except as a catalog listing. Replace parts as needed.

http://www.osengines.com/manuals/index.html

Download the OS manuals for their FS-26S-C (near the bottom) to get an idea of what you are looking at.