I've got an '82 300cd with pretty serious blowby. The filler cap dances around violently, and if I don't make sure the dipstick is firmly seated in the tube the crankcase pressure will push oil out of the sump. After about the third time this happened I replaced the O-rings on the stick -- that seems to have helped.

Still, the car starts easily down to zero F without a block heater (it doesn't have one -- the car is originally from southern California). The power is fine and the engine idles smoothly and barely smokes. It burns and/or leaks about a quart of oil every 1200 miles.