Onan 4000 ky generator has oil in the air filter

I get that on my 25 horse Kawasaki lawnmower engine as well. The bottom of the cast aluminum plenum that feeds the dual throat carb always has an oil puddle in it. I may try the modification on it in as much as the design is very similar. Never give it much thought. Never consumes a lot of oil between changes anyway and oil cost is inconsequential in reality.