I have just checked my manual (owners and service) here is what is says:
Engine oil and filter change after first 50 hours after first use (original owner of unit first placed in service) Then after 150 hours unless, operating in a dusty environment, hot weather, and once a year. I checked my post above, and realized I mistyped myself. I change the ISL-400 and the Genset Each August of every year for the oil and filter. I change the fuel filters on the genset every other year, unless I have picked up a bad load of fuel. I treat every third fill up with power service fuel treatment (red cap) and if it's going to be cold, I treat every tank. I never let my fuel level go below 1/2 full/empty (depending on your point of view), that is what we did on our traveling construction crew when I was building substations, it's a good rule to go by.
FWIW- The owners manual can be downloaded from onan's website, if for some reason you cannot get it, send me a PM, I will send you the PDF. It may be on the ACA Tech Library. It covers all the HDKA-H,J,K,T,U,V models, the last letter denotes which output you have I believe.
Monty & Janet - 2007 Alpine APEX 40 MDTS
S/N - 75715 - Retired - Master Certified RV Tech