Low oil pressure is an indication of excessive wear in the main and rod journals, exccessive wear caused by high mileage. The oil clearance is too large causing a larger volume of oil to pass through without building adequate pressur to create the necessary hydraulic buffer needed between the operating surfaces of the crankshaft, connecting rods, cylinder block etc...There is nothing that can be done about it short of a rebuild. Sludging doesn't affect oil pressure unless it's blocking oil passages or jamming check valves etc... Then you would have oil starvation in certain parts of the engine, in my experience this is found primarily the cylinder head camshaft journals. Sludging is also created over time, he is going through oil at a quart per week. It is either being left on the cylinder walls and burning off in the combustion chamber, leaking through the valve guides/seals and burning off in the combustion chamber or the engine is leaking it somewhere he haven't seen yet. He will figure it out, he just got it...

The engine has a very limited life expectancy IMO, time to swap it out.