battery drawi 1996 e 300 D
 

since it got cold the batteries been low in the morning.. had to put it on charger . I had autozone run their diagnostic on it and they said there's nothing wrong with the battery there's nothing wrong with the alternator .
the battery was an econopower refurbished one .
so I went to this place that sells batteries taken from a wrecked cars and got a beautiful three months old bosch.
he check the old battery and says it passes the load test and specific gravity is good there's nothing wrong with it .
so he put his meter on the battery and tells me to start the car and do a couple things and we have these results everything off battery at 12.7 v
key and starting position 12.7 v
starting the car drops to 10.5 volts
revving the car to 2000 rpms goes to 13.5 volts

What's your question?

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.