Delvac 1 is synthetic; Delvac 1300 is not. Both are diesel-rated oils. While the silver-jugged Delvac is RARELY sold in stores, the dino Delvac is quite common. The Mobil 1 Truck & SUV 5w-40 is the right stuff and is commonly available.

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'91 300D 2.5 Turbo 330K
'00 VW TDI Golf, 190K
'67 BMW R50/2
'73 Norton Commando Interstate