You can always tee in a mechanical gauge for comparison without making a permanent installation, but since your cold start pressure seems on the low side to me, I'll speculate that your stock gauge is pretty accurate.

 

 

What you could do is change oil and filter with a straight grade SAE 30W detergent oil (as a baseline), and with a mechanical gauge tee'd in the line with your sending unit and compare the two at cold and warm idle speeds. That will pretty much tell you what you want (or not want) to know. With this scenario you can also see what differences the multi-grade oil with additives is giving you. Save your used drained multi-grade oil for reuse. I do not recommend using any oil additive because in some cases they void the oil's original additive package.

Here is a viscosity comparison I made several years ago for another forum. Kinda unrelated, but you can observe how the different oils react to different temperatures.

 

The stock gauges are not accurate at all in my experience. Mine reads 35 -40 when it's actually 50. At hot idle it reads 15 when it is really 30. Verified with a calibrated mechanical gauge.

 

 

You guys are lucky that have stock oil pressure gauges! From 54 on, the F100 trucks got idiot lights for oil pressure and generator charging.

F250 and larger trucks like 54 Wrecker kept the guages.

 

 

The pan is so easy to drop on that truck. IMO, you don't want to free up and circulate any trash that may be in the pan thru the lubrication system. Drop and clean the pan first.
EditL If memory serves me right, the oil pump is outside the pan on that engine.

 

I would save the expense of using ATF as it has no detergents and is basically ISO AW 32 hydraulic oil with "stiction" additives and red dye. If you want to go the added expense use a regular snake oil flush additive like Rislone, STP, or Ravenol. And I would do as Ray suggested first. Drop the pan (save the oil for flushing).

 

 

Before the days of ULS diesel that would have been ok at no more than a 15% dilution. But with today's diesel at ~50ppm sulfur it won't help much. A 50% dilution is asking for trouble. I would just use the snake oil. You can inspect the oil pump if you want, but I think you'll find there is nothing wrong with it.

 

Are those pressures at idle? If so, they aren't a big cause for concern in my opinion. What is the pressure at cruise, when fully warmed? The rule of thumb is 10 psi per 1000 RPM, so you likely only need 30 psi at highway speeds. If you don't hear the rods or bearings knocking, clean out the pan and drive it! Being a Y-block you may want to check that oil is getting up to the rockers.