Using oil issue

An engine sitting for a very long time, if not properly prepared, can literally let the cylinders with open valves " breathe".
Suck in cold damp air, expel warm air, day after day can form rust patches in those cyls.
Lotsa these engines will seize, others will run with reduced power? from these cyls, and only discovered at tear down.
Not saying this is the case, BUT it is one of the reasons I rotate the crank a few times a year on racecar engines, (should do all).
A bent pushrod or stuck valve could cause a similar condition.