my 300TD is using oil

Hello everyone, I have a Mercedes Benz 300TD with about 190,000 miles on the odometer, and probably just over 200,000 real miles (odometer was broken for a few months). I have owned the car for about 3 years now and the car has a number of problems but the biggest is its oil habit.

The car has always consumed between 1 and 0.5 U.S. quarts between fill-ups. It seems to consume less oil when the majority of driving was done on surface streets. Freeway driving “seems” to increase oil consumption slightly. Initially, the back hatch would be decorated with small spots of oil, but upon replacing one of the oil hose from the filter to the cooler, the valve cover gasket (also adjusted the valves, been about 2 years since that time), and the sump gasket, the black spots have cleared up. However, the underside of the vehicle on the driver’s side of the drive-shaft continues to be wet with oil. The car smokes at start-up and when the accelerator pedal is depressed. It also smokes a little when its at idle even when warm. The smoke appears to be white or gray, not black. A recent compression test yielded average diesel compression in every cylinder (I don’t remember numbers, just that the values were good). Once, parked nose up on a steep driveway, a stream of oil poured from the interface between the engine and transmission. The bottom of the engine is always wet with oil, so it’s tough to tell what’s leaking on the engine. Parked, the car only spots the driveway, never leaving puddles. The car starts easily, once the glow-plugs are warm (first start takes time and is noisy, but every start after is smooth and quick).

To sum up, it has good compression, runs strong, but smokes and uses oil like mad. My first impulse was to replace the rear main seal (wet underside is a strong indicator oil loss is through a leak), but I have been told that both the timing chain and valve guide seals may be contributing to the picture. Should I jump in and replace the rear main (job looks scary)?