The experience I had was with a 400 engine in a pickup. When I bought it used, It had low oil pressure when hot and at idle (10 PSI), but, otherwise it was quiet and ran fine. Even though the truck only had 70K on it, I figured the bearings must be worn enough to cause it. Pulled the pan and everything looked fine. Even Plastigauged them to be sure. Next, I pulled the oil pump and checked it's gears, housing, and wear on the plate. All good there too. When I pulled the pressure relief valve apart, I found a wire (probably from the oil strainer) stuck in the bore of the relief valve, not letting it close all the way. Pulled the wire out, inspected everything else, declared it good, and put it back together. Normal oil pressure was restored! This 318 could have a similar problem with the pressure relief valve stuck part way open. When cold, the oil is thick enough to restrict flow back into the pan, and still allow decent pressure to the rest of the engine but, when it's hot and thinner, the oil pressure would drop as more oil went out the bypass. Given all the crap that was found in the oil pan, a stuck valve is not much of a stretch!
