As cozy said, I would not trust the stock gauge either. It isnt really that trustworthy, second though, and Im remembering this from a post on Zcar or here a long while ago.
I believe the member was Lifergaurd or something to that effect, that actually had taken the lifters apart, throughly cleaned them, replaced the old springs with new, and the put them back together.
Im sure if you do a search on here or zcar it should come up. I do know that you can take them apart, and clean them, its hard to find the proper spring for the lifter but it is possible.
I did have a P90A that I was going to use for a 240Z ages ago and did do the cleaning. Depending on the PO, they can become quite filthy.
Tearing them down though is easy as is cleaning and when you put them togother just make sure you put a bit of oil in them.
I never got to try the whole thing out as I sold the motor, but its worth a try before removing the whole head.