Another reason (that I have recently discovered) for a real oil pressure gauge, is that it will let you know if a piston cooling nozzle has fallen out.
Or worse still, if the oil pump pickup tube is cracked.
In my case the oil pressure was good with the engine cold. As the oil got warmer and thinner, air started to get sucked into the tube giving lower oil pressure until the pressure dropped to around 5psi gauge. I was fortunate as I had a pre-lube oil pump installed and was able to drive with it switched onto give me around18psi. It got me home with careful driving and then the diagnosis had to be done. Eventually got the problem solved and oil pump, pickup tube, bottom and main bearings all replaced. The problem was spotted before and real damage was done. This was around 1994 - truck out of warranty of course.
It was common with the earlier 6.9 engines (in my case 1986) for the pickup tube clamp fasteners to come off while engine ran. This allowed the pickup tube to vibrate (no longer clamped tight) and eventually crack. I was lucky as mine cracked about halfway around and did not completely break in two.
The repairs I had done at a IH truck place and the bill was around $1800 including tax - I then lived in an apartment building and had nowhere to work on the truck.
The truck engine now has around 350,000km and seems just fine.
So the moral of the story is get a mechanical lube oil pressure gauge fitted - it just might save your butt like it did for me. Very cheap insurance you can't be without.
HTH
