I had the same problem when I purchased my '81 300SD about 8 months ago. I upgraded from the round plastic tube that goes from the valve cover to the oil separator to the flat one that has baffles in it. I also sealed the separator with RTV black. The results were good, and even though the RTV black wasn't able to hold for more than 6 months, it was easy to clean off and do the job again.

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'81 300SD