be going or gone. The ones in my 2002 Boxster "lasted" to past 250K miles but not everyone gets that many miles from them. Also, they were probably bad before 250K miles but I only had them replaced when the tech pointed out how they were leaking oil. As I think I mentioned the tech told me the old o-rings were in pieces and those sections of the o-ring that were missing ended up in the oil and had done so probably long before I had the o-rings replaced. Had I bothered to check the oil filter for trash/debris -- I have had the oil changed for the last couple of years no longer having the time to do this myself -- I might have spotted this stuff in the oil filter.
If a hydraulic piston o-ring is failing this can result in cam timing variability. If the debris is from a valve guide seal oil consumption can go up. The engine can smoke at times other than cold start up. If you are able to see the valve stems -- that portion that extends down into the port -- you might see signs of oil on the stem. On an intake valve the oil would be wet. On the exhaust one or more valves could have some carbon build up over and above and beyond what other exhaust valves have.
Trouble is getting a good view of these. If you remove the intake manifold you could set yourself up for some work getting the thing resealed. And all the rubber hoses are probably fragile and you could create a vacuum or air leak. Even removing the exhaust manifolds is a bit of work.
I do not recall if any of the sealed surfaces are sealed with a green sealant. The oil pan is sealed with a black sealant at least the few I've seen. Ditto the cam shaft covers. But if a green sealant is used there is often some extra that gets squeezed out and if this extends into the engine and the violence of the oil will have it come apart over time and bits and pieces of this ends up on the oil filter and oil filter oil. This sealant though is pretty soft. O-ring or seal material a bit firmer. And valve stem seal material is probably the firmest of the material.