diesel oil requirements in the 1980s

hey guys received my manuals the other day. well written and helpful. i guess i got lucky with my 240d buying it with limited MB knowledge. i do have a question as to what types of oil was mercedes requireing for oil changed in the early 1980s. my owners manual states that the car had regular maintanece (it shows 73k on the odometer but is has quit working) the oil changes were performed with castrol 20w 50 not exactly a diesel motor oil. these were done at authorized service ceneters . was this the best choice for oil back then? could this be the reason i have a lot of crank case pressure on an engine with less than 100k? i have the paper work and it shows the milage at 67k in 2000 and the car sat for a long time so i am confident that the engine has less than 100k probably more like 85k tops thanks ASP