If you suspect any internal engine wear, you should do a compression test first to identify any suspect cylinder which has a large difference in compression to the other cylinders. If there is one, then you do a leakdown test on that cylinder to ascertain whether it's leaking out the rings or valve(s).
My understanding is that if air is leaking back out through the intake, it's intake valve, if it's leaking out the exhaust, it's exhaust valve and if it's leaking back out through oil filler, then it's piston rings.
Maybe some of the more experienced would chime in to verify this
'89 911 Turbo Cab
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