Could be you have a bad turbo seal causing reduced turbo performance and blowing oil into the intake manifold. Does the turbo shaft spin freely? It shouldn't freewheel but there should be minimal resistance, barely perceptible raidal play and absolutely no axial play.

On other cars it's not unusual for the turbine housing to glow red after a hard run. If it's smoking I imagine there's an oil leak or it has a coating of inappropriate paint.

Sixto
93 300SD