Just check your castings and oilways as I have seen some which have not been fully bored through. There are also oil drip resevoirs which can be badly cast, although I suspect the main problem ist poor pressure to the tappett causing it to no longer cope.

If you want the ultimate go roller rockers and roller tappets on a good cam you should see it lasting forever (a friend runs this and has raced it for years). The standard setup eats cams if you like your revs.

Anything is worth a try as cams cost lots of money.