New to AC with a WC, rocker shaft / oil pressure

I bought my first ever Allis Chalmers which is a 1936 WC. It appears to have a 1947 electric start engine in it. I got a great deal on it and it's in nice condition and runs great.
When it gets warm the oil pressure gets a little lower than I would like. I'm running a Fram filter with straight 30 weight oil. I tried switching back to the Carquest filter that was on it with no change. The filter standpipe is in place. I added one washer under the oil pressure relief valve cap and did pick up some pressure. I think a switch to 20w50 oil may be what I need. But I also recently learned that the rocker arm shaft is somewhat pressurized and sealed on the ends with cork. I'm guessing I may need those corks replaced. Would this be responsible for the lower oil pressure? I have a parts book and have the part number for the corks but nothing really comes up online. Are people just making these? It seems like driving in a freeze plug would be a better idea. Anyone do this? What size? Any other comments or suggestions are welcome. Thanks.

When I had my WD45 engine apart last year, I think I used tapered rubber plugs in the ends of the rocker shaft instead of corks. I think I had to trim some off of the smaller end because they were long, but I oiled them up and they pressed right in. Still use the cotter pin behind them to keep them from blowing out. Not sure what the real manual calls for, but the I&T manual says that normal oil pressure is 11psi. Or, like my operators manual says, somewhere between the "O" and the "M" of "NORMAL" on the gauge

Oil pressure is on the M when cold. Adding the washer took me up to the A. But when warm it drops to the line at the bottom of the normal range. Slightly below at idle.

If you're corks are bad you'll know it. When running, you'll be able to either see oil running our the end of the shafts past the corks or no oil will come out there becasue the corks are doing their job.