Diesels take a good battery to start them in cold weather due to glow plug draw, high compression, and high oil viscosity. My '97 E 300 D calls for an 100 Amp Hour battery, which is a fairly large battery. In really cold areas plug in engine heaters are needed.

So if your battery is large enough, try a synthetic oil with a cold viscosity reading such as 5W40 or even 0W40. Either oil can be used year around as they are 40 weight at operating temperature.

Also, I believe that with a fully charged battery your charging voltage should be near 14, but just after starting it could be lower while the charge is being put back into the battery.

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'97 E 300 D