You can soda blast them and they'll be good for another go around.
How do the look when you take them out? White and dry, black and wet?
You could have a leaky crank seal, filter grease melting and getting sucked in, too much oil on your filter, too lean jet main jet or too rich main jet.
It's hard to determine, but my bet is that your crank seal is weeping, had the same problem with my 1990 at it's 10 year Bday