Just my observation: The D2 5J had a high pressure injection pump, I believe. I've never seen the figure for operating pressure. This was the D2 of pre-WWII. With the model change to the 5U came an improved pre-chamber design and a much smaller injector about the time war rationing ceased. The 5U runs with no problems at all on waste vegetable oil, and I am told it will run on anything you can pour into it, oil wise. I just have to keep it warm enough to flow.
Is there a connection between wartime research on all=fuel and the delivery of the improved system on the 5U?