The Ancient Greek. The idea of placing the camshaft at the top of the engine is OK in a high usage engine but if left a couple of days all the oil drains and it starts to rust then high wear occurs on the cam lobe, valve lift goes down and the climb goes down and the take off run increases happens very frequently. Next problem is the valve gear design from 1929 Cadillac side valve engines which are splash oiled from the crankcase.
To little oil makes it to the over head valves to cool them and it cooks on the valve stem causing it to stick either open or closed. They have problems with the wrist pin in the pistons which use an aluminium bronze plug in each side of the pin to centre it when they wear and role over scratches the bores, first indication is ali/bronze in the oil.
Centre bearing on the crankshaft can spin. Cooling of cylinder heads can be a problem Lycoming max tem is far to high at 475f should be 380f for long life. At their tem limit in time the oil control ring will anneal the spring tension will go and it will rollover and break with huge increase in oil consumption and detonation in that cylinder.
I have seen these problems many times and paid the bills the Lycoming is a piece of junk as far as I am concerned. take care.