From oiling diagram that is last in line for that side of engine and feeds second to last main bearing, perhaps your bypass is kicking in and not enough oil pressure to fully pump up those lifters and ejecting oil up easiest route.
You would think if that Main spun you would still have oil pressure so I would look at pump first or possibly gasket in timing cover, I am limited in my Buick knowledge, but would think if reading pressure up front and oil is reaching back, you should see higher pressure with either collapsed lifters or spun main, my 2 cents since no one chimed in yet..
Was any work done to engine, have you have cover off or pump apart, was it making noises or just saw the low pressure and pulled and put on your engine stand?
If just rebuilt and everything done correctly, 1st cam bearing installed properly for oiling, hear that causes lots of issues if not placed in properly.
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