I have the Onan Marquis 7000 and even after running it for a few minutes, couldn't get 2 quarts in (Wix filter, Mobil 10W-40 and Champion plugs). PO used it a lot, 500 hours in 6 years, I haven't used mine yet in a campground, just as a backup genset for use at home during 2 power outages, 50 hours total in 2 years. I bought one of those plier style wrenches.
Ole and Anne Anderson, Highland, Michigan
'02 Adventurer 32V, Ford F-53, ours since 4/08,Goodyears, Konis, SeeLevel, CHF
'84 CJ-7 , 5.3 Chevy, 3" lift, 33's, Detroit Locker, Fiberglas tub, winch, hi-lift