My Generac is showing a message to change oil/filter. It's been in there for 2 years exactly. I don't know how many hours are on the oil but it looks like new. It excercises for 20 min every week. It may have run for a few hours during power failure. It's Castrol 0-40 and a Fram TG filter. Should I change it due to age or just let it go? Runs on porpane.