20% is actually alot, it could be worn rings, a valve not seating correctly or a blown head gasket between #2 and #3 clyinders.

Other things that would be of help are, is the car blowing smoke, if so what colour, white gray, black? Are there bubbles inthe coolant ot a oily film floating on top of the coolant when you lift up the cap, is there a milky substance on the under side of the oil filler cap? Does the engine use alot of oil?

08' Jeep Wrangler
64' Plymouth Sport Fury 383
58' Triumph TR-3A