Your SD has the same engine that is in my TD. I put 2 gallons (8 qts.) of oil in at every oil change. I buy Delo 400 at Walmart, comes in 1 gallon jugs. I also change the oil when it's hot and wait for the oil stream from the pan run down to a trickle before putting the drain plug back in. Add my 2 gallons, start the engine and let it idle for a couple of minutes, then shut her off. While the oil is draining back into the pan, I clean up all my rags and tools, then check the oil. It's always been at the full mark.

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1999 MB SL500 (110,000 mi)
2004 Volvo V70 2.5T (220,000 mi)
2014 Tesla Model S 85 (136,000 mi)
MBCA member