I use Rotella 15W 30. The D2 holds 14 quarts. You'll need more for the Air Cleaner, Starter Pinion, Injection
Pump Housing, etc.
There is a thread on this page on "D2 Oil Capacity" with a couple of links in it on further oil discussion. I use straight 30W Rotella in my D2-5J. For the Air Cleaners I use 5W30, ATF in the Pony Clutch and "Non-Detergent 30W" in the Injection Pump.