If it were me...
I'm not sure if this is Cummins/Onan dealers nationwide of just my dealer, but take model number and serial number to a dealer of your choice and do two things.
1. Have them enter your data into there data base so they can contact you if any recall notices and also when getting any parts all they have to do is look your info up. And when there ask to buy an oil filter, which will solve your problem.
2. My 4ky same size but older does not have any filter. And I asked the same question first time I changed my oil also.
