I do all the PM on our generator. It seems pretty straightforward. The price for a "service contract" from the vendor was up in the $400 region, and basically covered only oil/filter changes as far as I could tell. What really frosted my cake was the clause in the contract that disclaimed all responsibility for any damage their negligence might cause-- so if they left the drain plug loose and all the oil ran out it's not their responsibility.
At that price I'd be expecting a real service contract where they fix it when it breaks (with priority in emergencies) as well as do the periodic maintenance.