I had the same issue on my 292. I used the Marvels mystery oil and a dead blow hammer. The other issue I ran into was a bad lifter on the same cylinder (#4 intake). My motor had set up for so long (10 or more years)the lifters would not release the pressure. Even after I freed up the valve, it kept bending push rods. I pulled the side covers and got new lifters. Worked out great!! Lifters are $3 a piece, x-number of pushrods at $1.50 per rod, didn't take long to realize the cheaper way, and of course the bending of the side covers. But mine did the exact same thing, so you may want to look at those lifters.
