Tony, this is strange enough to make me curious

Just my 1-3/4 cents... if your plugs are tan and you’re not seeing a lot of smoke (1 quart per 100 miles would generate noticeable smoke), you’re probably not pulling oil through the cylinders and burning it (through the intake or otherwise). You’ve got to be losing the oil on the exhaust valve side. Sounds like bad valve guide seals or clearance problems on the exhaust side. You said that the pipes were sooty… are both sides equally sooty?

Dan J