I have a 82 300TD and have replaced the SLS pump gasket. It is available from the dealer.

If your getting a lot of blow by it could be pressurizing the hydraulic pump. that is the only place the engine oil meets the SLS system. The gasket is not the problem. It would be the pump.
I just don't see how the oil would enter the pump. But I guess it is possible. I did not look at the pump, trying to figure that out. The pump rear just has a drive shaft setup.

http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1UJ057HUE1UJ05JUKC&year=1982&make=MB&model=300-TDT-001&category=L&part=Suspension+Pump

Consider flushing the SLS system and installing a new filter and see what happens to the fluid. It may be that the 2 oil levels are coincidental. Drain the SLS system at the rear and renew the filter. Add new Mercedes hydraulic fluid and see what happens. If it still happen then consider finding a used pump. It's $940 for a new pump. That's expensive.

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1970 220D, owned 1980-1990
1980 240D, owned 1990-1992
1982 300TD, owned 1992-1993
1986 300SDL, owned 1993-2004
1999 E300, owned 1999-2003
1982 300TD, 213,880mi, owned since Nov 18, 1991- Aug 4, 2010 SOLD
1988 560SL, 100,000mi, owned since 1995
1965 Mustang Fastback Mileage Unknown(My sons)
1983 240D, 176,000mi (My daughers) owned since 2004
2007 Honda Accord EX-L I4 auto, the new daily driver
1985 300D 264,000mi Son's new daily driver.(sold)
2008 Hyundai Tiberon. Daughters new car