Piston rings, valve guides, valve stem seals. You need to do a leakdown test to figure out what it is. Do a quickie compression test to possibly discover if it's just one particular cylinder causing trouble but a proper leakdown test is in order for each cylinder.

1950 Chevrolet 3100 (Ol' Roy)

1939 Packard Standard Eight Coupe (The Phantom) | 1956 Cadillac Coupe de Ville (The Bismarck) | 1964 Ford Galaxie 500 coupe |1966 Ford Mustang (Little Red) | 1969 Pontiac GTO (The Dishpit) I 1979 Ford F-100 | 1976 Ford F-150 (Big Red) | 1995 Ford F-150 (Newt)