I have a very similar situation with my '89 740T. With about 315,000km or 196,000 miles, it recently tested high for HC ... and it has fairly normal oil burn. But I get about 450km, or 280 miles, per tank of gas with a manual transmission. Poor engine timing and leaks in the inlet air system could result in high HC and lowered gas mileage (kilometerage?), so I'd recommend starting there on your car.

If those two are good, then I'd test what I suspect is wrong on my car: the oxygen sensor or the catalytic converter. You can self-diagnose both the sensor [http://www.brickboard.com/FAQ/700-900/EngineSensors1.shtml#Oxygen Sensor Life and Diagnostics] and cat [http://www.brickboard.com/FAQ/700-900/Exhaust1.shtml#Catalytic Converter Diagnosis], but I'm going to let a professional diagnose and replace the cat. By the way, after my car tested high, the guy at the emissions test station recommended replacing the cat. He said every car with that kind of mileage either needed or already had the cat replaced. I'm the second owner and not really sure if/when it was done before.