I don't believe that there is an operating oil pressure listed in the TM. That's why they have the low oil pressure switch. There is another forum called "smokstak" that deals with antique engines, and has a section devoted to MEP's and military gen sets. You might ask there is you really want to know. There are some really informed people on that site.
However, when mine are running after about 45 minutes the pressure is still up around 35 to 40 psi. I never fully trust pressure gauges, in my line of work, I've seen way too many of them read wrong. I would think that the pressure should be around what any other engine would require for lubrication and cooling.
Your question brings to mind something I have been wondering for a while now. A little while ago someone posted about having a new (to them) mep, that had a drain valve for the engine oil (which they all do have) and thought something was changed or done to it because of this. Another member responded that they thought the drain was there in case the troops had to leave one in a hurry, they could open the valve and leave it so it would destroy itself before the enemy could make use of it.
I have two thoughts about the validity of this, and I hope someone can chime in and clear them up for me...maybe someone who was responsible for these sets in the military or worked on them.
1) as long as the hose is hooked to the frame of the set like it is when being stored, the oil would take forever to drain down enough to do any damage. I wonder, if the statement about the valve were true, is it common practice for the military to run these sets with that hose disconnected in case a fast get-a-way is necessary???
2) what advantage would opening the valve and draining the oil from a running set be to the troops having to leave it and destroy it if it has a "low pressure switch"??? Wouldn't the switch just shut off the set before it totally destroyed the engine? And if not, then why have the switch there to begin with?