Laze the exhaust manifolds with a temperature tool see that they are all getting to the same temp and there are no cold ones. Also can help find exhaust leaks. (as can a Carbon monoxide detector in the cab while driving, lol)

Lifters can get a little bit of junk in them and it can plug the oil hole and cause the lifter to not pump up properly. Running some seafoam or such in the oil and doing a couple oil/filter changes can sometimes free up that crud if it's a composite of smaller particles.

If ya do have to open the valve cover, adjusting the valve lash may be a thing to do. Could be just time to do em.

If you do need to pull a lifter, there is tool for that that grabs the inside lip of the lifter and yoinks it straight out. Don't mix them up if pulling more than one as they wear in to the cam over time and it's better to put them back in the same hole if not replacing.

Engines are pretty tough in general, but sometimes the little lifters just suck up some random garbage. I've had it happen on relatively new vehicles, tick for a while, and then just mysteriously stop and never happen again.

Can be frazzling to stress over but again, it was just some random junk that a filter caught on it's next pass.

filters don't generally filter all the oil, sometimes things bypass it or find a little spot in the engine to hide for decades.

Good luck, please update us if'n'when ya figure it out!

Cool car, love me some Olds. :D