I have no problem letting my OM603 idle in the morning (10 to 15 minutes) to warm up. Being in the marine industry, we have ALL kinds of diesels that idle many hours with no issues. Of course none are MB's. Cat, Detroit, Volvo, Man, MTU, Perkins, all spend lots of time idling. I think a well maintained engine should be fine. I have toyed with the idea of a fast idle solenoid or cable to raise the rpms up to 1500 or so to promote faster warm up. Of course the block heater helps alot. At 14 degrees this morning my MB fired right up without the block heater. Just takes a while to warm up.