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.
1994 MB E320 Coupe (199,500 mi)
1992 MB 190E 2.6 (173,000 mi)
1983 Porsche 911SC (148,000 mi)
2004 Volvo V70 2.5T (198,000 mi)
President of CT/Westchester Section