If I was driving the car everyday after a 10,000 mile overhaul and a lifter went bad id put in a set of lifters. The 15 years kind of change things though. have you tried adjusting the valve? Sometime backing off the adj. will loosen them up. id add a lifter additive, adj. the valves and see what happens. oil pressure is not the best in the world, but ive seen less. if its low oil pressure causing the lifter noise the new ones want help. it dont have to be the same amount of oil out of ea. rocker as long as they are all getting some oil. sounds like the cam is ok from sitting up. if it was mine and i thought lifters would fix it id put lifters in .