Gray Smoke After Coasting
 

My 1981 380SL emits a puff of gray smoke, particularly after coasting on to an exit ramp and accelerating after driving for a while. However, it doesn't smoke while driving, and the car runs great. So I'm assuming the compression is good.

Coolant and transmission fluid levels are constant, but the car is using some oil. So, I suspect that it may be the valve stem seals again, even though they were replaced when I had a reputable shop do a valve job less than 50k miles ago.

Is it common for valve stem seals to go in such a short period, or should I look for something else.

Thanks in advance for the advice.