Universal 25 oil leak via air filter

Hi ! I'm OK as far as sailing is concerned (Just sailed my Ericson 34 -11 days, alone- from Beaufort NC to Sint Maarten, Dutch West Indies) but I'm almost ignorant as far as diesel engines are concerned... Here's my problem: My faithful 1987 Universal 25 -with around 1500 hours- leaks oil through the air filter ??? How come ??? What should I do ??? Thanks for your help