I use DELO 400 (in the blug one gallon jugs from Costco) for the 617 diesels, and Amsoil 15W40 HD Synthetic for the 603 engines.
I think DELO has added something about synthetic to the 15W40 oil but I figured it was hype and not a full synthetic at that price. I am suspicious on every claim that manufacturers make these days. They are getting clever as all they want to do is SELL SELL SELL!

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!