Has the oil pan ever been off for a connecting rod clearance adjustment? The most common cause of low oil pressure on the splash-oil engines is wear on the connecting rods. It's a pretty simple job to drop the oil pan and take a shim or two out of the connecting rods, just make sure you don't overdo it on tightening up the clearance. I use a product called "Plastigauge" to set the clearance to about .0015".
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
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