It's good to always post the Model Number and Spec Code for an ONAN Generator. And you did, and KY Series has NO Oil Filter. We have a much older 4KY and I wondered the same thing. Turns out 4KY has a Splash Lubricated Engine. Like many mower engines. It's also a high-rpm engine, and it's air cooled. It has to have breathing room in its installation, BUT it MUST have its Access Panel in place! Leaving that Panel off breaks the cooling air flow pattern.
Bigger ONAN's, and older small ones in the 4KW range, have twin-cylinder, low RPM, pressure lubed engines and those have oil filters. Not 4KY. May sound cheesy, but they'll still run a long time. Just use good oil, keep it full, change it often, and let it get air.
OH! Exercise it as prescribed in the manual. And, use stabilizer like Startron or Stabil, in all your gas.
EDIT: The post above included a link about 4KY, and within that link was another link, to the
ONAN 4KY Operator Manual. I looked and it's pretty detailed.
Earlier versions of 4KY had a "Low Oil" cutoff switch. If it thought your oil was low, it'd stop the generator. Apparently it kept more engines With Oil From Running than it saved engines with Low Oil, and it's been removed from the 4KY series for years. Ours is Spec "K" and it was gone before then, and you're up to Spec "P!" So if yours doesn't run one day, somebody'll say "Low Oil" or "Bad Oil Switch" and it won't apply to you. I say that thinking (but not verifying!) that ONAN hasn't found a way to add a switch back in there again... It DOES mean all of us with 4KY's need to pay attention to the Oil Level, and that Manual says how to check it.
