Few "bugs" to work out.
Surely will be handled and steps closer to enjoying all this work !!!
Best of luck sorting it out :)

What can be going on here?

1. SAE40 oil sheared down from idling the engine for 3 hours. Highly unlikely.
1. Oil was always that light. Previous owner ran Prius spec 0W16 for greater fuel economy. Highly unlikely.
2. Something, most likely diesel fuel, has diluted the oil. A leak somewhere would explain the low oil pressure as well. More likely.

sae40 is a lot thicker originally. diesel leak in oil is possible because injectors are inside valve covers but oil didn't smell like diesel at all. Usually oil smell diesel if there is fuel in oil, except biodiesel but I don't use it.
Maybe tomorrow I will change oil and do more testing.

Paper bag or something like that dopped in the oil...fuel thinned oil will wick up the paper a lot more. Just a rough field test. A proper oil sample to a lab would be the best COA.

An insane amount of 'changes' to the engine, very difficult to see potential isseus when you're in the system so to speak. Getting her all functional may take some trial and error. Nothing wrong with that. You end up with a beter product.

Check out BlackStone oil analysis.

Injection pump leaking diesel into the crankcase?

fuel lift pumps are electric and external, injection pump leak to oil is possible but not likely. If something leak I bet it will be ijector return line leak inside of valve cover.
30C temp is continue but I think do oil change later today.
Now just swimning and chilling

https://cimg7.ibsrv.net/gimg/www.off...123430dfc0.jpg

I was just a gonna say drop some oil on a paper towel,fuel will wick out.

update. yesterday I change oil, it have diesel in it so some injector line, return line or something leak fuel in oil. But another problem found when I cut filters open, by-pass filter have collapsed because pressure difference, wierd.

https://cimg5.ibsrv.net/gimg/www.off...b4df4bed76.jpg
https://cimg1.ibsrv.net/gimg/www.off...914051ca09.jpg
https://cimg3.ibsrv.net/gimg/www.off...8b76a42940.jpg
main flow filters, two pcs, are 21 microns. this by-pass filter is only 5 microns and I have modified filter housing to feed turbos and injection pump from this filter. But there are some mistake that I think all oil go first to main filters and after that to one 5micron filter. I measure pressure only after filters so pump can be against catastrof valve like 15bars and after filter pressure is 3bars. I turn and weld bypass filter delete part (dummy filter?). pressure go up for little time but in test drive it drop again, I think fuel get in oil again. I hope this hot season end and I can work more with boat soon.

This may take some time to solve.

Better buy a drum of that SAE40 oil.

What is the oil volume of the Transmash?