If your coolent is clean and your not losing any. As well as your oil being clean, I would take a shot at saying its either valve stem seals (easy replacement if your mechanically inclined) OR bad rings on a cylinder. Check for fouling plugs. I notice most valves seals that go bad are on the exhaust side so you don't really foul out plugs. But if your fouling a plug it could be a bad ring.

Richard E
1989 Montero - Stock-ish
1989 V6 Auto Raider - 5.3 Vortech Swap.
1987 Mitsubishi Starion 2.6t, soon to be 3.5
1983 Honda XL600R