The M103 will often develop a leak in the head gasket at the passenger side rear of the engine. Since the engine is tilted slightly to the passenger side, the oil leak, if present, will drip off the engine right there and not dribble down the side of the engine. And it will only drip while running. So you won't get a puddle below and you won't get a oil streak on the engine. So you have to check it like Phalcon51 suggested - with the engine running.
I'd also suspect valve guides/guide seals by 150K miles.