High mileage small block Chevy engines have a chronic problem with worn camshaft bearings causing low oil pressure at idle. That's been the case since the late 1950's when the 1955 265's started needing to be overhauled, and things haven't changed since. Our family shop stopped doing "in the frame" ring and rod bearing jobs way back then because too many engines ended up with a flickering oil pressure light at idle.
There are three ways to solve your problem- - - -do a full overhaul including changing the main and cam bearings, which should have been done while you had the engine out, run heavier-weight oil like 15W-40 or straight 30 weight, or install a high volume oil pump to overcome the leakage past the worn cam bearings. I'd go with heavier oil- - - -simple, easy, and no downside unless you're starting in extreme cold weather with cold, thick oil. (Like below zero)
Edit: A Melling M-55HV high volume oil pump costs about $60.00 with free shipping on Ebay. It will make the problem disappear temporarily, sort of like giving a patient a big dose of aspirin for his headache, but it won't cure the cause of the problem, his brain tumor. It just masks the symptoms. NAPA oil filters are made by WIX, with the only difference being one digit on the part number. They are far better than most other filters. Use their "Pro-select" brand filter that big fleet shops use, unless you like paying extra for a little bit of wrinkle paint around the bottom half of the filter and a different color paint on the can.
Jerry
