40 psi IS low on start up and cold oil. It should be 60psi. There is either something wrong with the oil pump or the oil is contaminated with diesel.
Now if the engine starts of with 60psi and goes down to 40psi as the oil is at operating temperature, that is normal.
But dont take my word or anyones word for it. Do your own research. Because noone here will buy you a new engine if you fire it up and poke a hole in it. Google specific pdf files for c-7 or at least 3126 engines.
A better idea than to skim through anyones list of opinions is to download the engines manual and read through that.
40PSI is a perfectly acceptable oil pressure at a low idle, still cool after just having started up. 40PSI is plenty of pressure to lubricate everything, and is not severe enough to be indicative of anything wrong with the oil pump or there being diesel in the oil. Yes, as the oil heats it thins, and pressure will drop some. There is also a good chance on these trucks that a low oil pressure would be the result of a bad gauge or sender, before the mechanical pump should be suspected of having problems.
I went and double checked the manuals, and they confirm. Both the military manuals and CAT manuals say minimum acceptable oil pressure at low idle is 15PSI. Minimum oil pressure for running under load is 36PSI. So even if some people say 40PSI is abnormal, it is acceptable, and sufficient to lubricate the engine.
As for the comment about not skimming through my document, this engine oil pressure question isn't in there, and that's not what I was alluding to. There are however sections about the air compressor brackets and oil tubes coming loose, cracking the front housing, etc., which are common sources of oil leaks for these trucks. That's why I was pointing him to it.