I've been reading the oil debate and don't want to start that again. Actually it's all very informative. I have a slightly different issue. I have a '73 220D which I've owned for 14 years. Used regularly for 10 years; sat up 2 years; changed oil regularly and always after it had been unused for a while; been using it a couple of times a week for the last 2 years. I now want press it back into more regular service. Drained the oil and the oil cooler. The oil is getting dirtier at oil changes; blow by? lack of use? I'm not sure why. I'm toying with the idea of putting synthetic in it to clean it out. Also, somewhere along the line I used synthetic one winter because it started hard and the synthetic clearly allowed the cold engine to spin more freely with less drag. I sort of want to experiment with Mobil 1 15-50 over against my usual Cast 20-50. I've always resisted changing oil brands (I've always resisted changing in general), but this might be an interesting time to try something different. Making some mistake that I haven't forseen? Cutting loose the sledge that's holding the old girl together. The new service involves a 300 mile trip to another location and leaving the car with my wife (who I still love) as her primary mode of transportation around town.
Thanks as always for any information.

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Dan Stadt
'85 300D turbo
'79 300SD
'73 220D