oil pressure and injectors?

Put an oil pan gasket on an older ford l800 today. Most of the gages don't work but honestly not sure of the oik presure gage works. It shows nothing. The truck runs good and made no noise after running for 10 min. I with smaller diesel engines oild presure is required to operate the injectors, is that the case with big engines also?

I think the engine is a 300 hp cummins from the late 80s.

 

No ideas? Anything will help. Trying to confirm it is OK to send this truck out tomorrow. like I said it ran fine for about 10-15 min but I want some confirmation without having to go back to work and pull the oil filter to see if it is pumping oil.

Thanks for any information.