I have an identical issue with my 94 sl-600. Haven't found the cause -yet. It DEFINITELY is engine oil 'pooling' in the intake manifold runner. How it gets there is the mystery. I SUSPECT the oil drain-back holes on the head are marginal or? and the PCV system sucks up a little bit somehow. I am gonna modify the PCV inlet with a deflector or ?
Exactly the same symptoms 20-30 min of operation, smoke starts, then will go away. Sometime worse than others. Seems is gets a slug. Engine operation conditions do not seem to make any difference.