A couple of quick thoughts.
1. The first engine I rebuilt was from a 1962 1.0L Austin Healey Sprite. The second problem I ran into after I tried to get it to run was that it would spit oil filters off. The original was a cup with replaceable element. Changed it to an adapter and a screw on. Spit those off, too. Hmmmm? I probably spent at least $1000 on the rebuild, in 1973, but didn't replace the $6.00 oil pressure relief valve piston. Unlike many US systems, the piston was separate, spring loaded, and part of the engine block rather than the filter.
Oddly enough, after I replaced that piston, which was visibly scored, the oil pressure dropped down from pegging the 80 PSI meter, to about 45 PSI. Hmmmm?
Many US built oil filters have an internal pressure relief valve. Way cool unless you are serious about filtration. What I recommend is a WIX 51794 on a remote adapter. These filters are two quart with no internal pressure relief valve. They fit the same adapter as a FRAM PH-8. Change them every time you change your oil, or every 3000 miles if you are using non-synthetic oil.
2. A 1953 PG engine should keep a 30 PSI Oil Pressure Gauge pegged most of the time when the engine is off idle. It doesn't appear that is a problem for you, but the drops in pressure should be.
When I pulled the oil pan off of my 4400 with 1954 235 in it, and external factory filter, I found about a pound of crud in the bottom step of the oil pan, where the oil pump inlet is. I had trouble getting it out with a 1200 PSI power washer. Other possibilities are leaves, bird's nests, fish, etc. I'd suggest that you spend an hour or so to pull your pan and see what's in it.
The connection between the tube that runs from the pump to the strainer is also critical. If that joint comes loose, you'll be sucking air, which has little in the way of lubricating functionality.
3. Road Draft Tube. I don't think so. My 3800, which runs . . . so far, has a nasty case of blow-by. It doesn't come out the road draft tube. Maybe because it is totally clogged, but . . . the oil filler cap looks like the "Little Teapot That Could" Seriously, puff, puff puff, and it isn't water vapor or unburned fuel. You should certainly clean your road draft tube, which, as I remember the maintenance manual, is supposed to be done annually.
How many of us have been driving around in trucks where this perfectly reasonable maintenance chore has been neglected for more than 10 years?
