oily air filter

I own a 1992 ford w/ a 302 and recently i have started adding oil between oil changes so i began searching for a cause. While checking the air filter i realized that the crankcase fitler was soaked in oil and has also covered my air filter box w/ oil. I have checked the pcv valve and it is clear. The truck starts and run great, doesn't overheat or have power loss. Any ideas on what would cause this large amount of oil to be expelled through this crankcase vent? What can i check or replace any help would be greatly appreciated.