Why is my f-150 302 blowing oil out the dip stick tube?

My 1981 f-150 4x4 302 engine was replace a few yrs back with a 302 eng w/16,500 orig miles out of a 1984 f-150 4x4 pu truck. I've driven for several yrs probably not accruing more than 25 to 30,000 miles on the truck and never had an oil problem til now. Out of no where it started blowing oil out of the dip stick tube. The tube was replaced just to make sure and was not the problem. In approx 1000 miles it blew out about 2.5 qts of oil.
It cranks and runs good and doesn't smoke out the tail pipe. A tiny amt of smoke thru the oil opening on the valve cover. There is a new pcv valve installed. I've talked to a local mechanic and he thinks it is a stuck ring but he isn't sure. Need Help!