Cylinders 1-4 have stuck or worn piston rings. Getting a 20 PSI rise by adding oil tells me the oil is helping the rings get a better seal momentarily. A head gasket problem would cause low compression on two adjacent cylinders due to leakage from one to the other, or there might be bubbles in the radiator and/or overheating from a compression/combustion leak into the cooling system. Before you take anything apart, try this- - - - -Change the oil, using any old brand of inexpensive straight 30 weight. Add 2 or 3 quarts of Diesel fuel or kerosene to the crankcase so the crankshaft is partially submerged. Start the engine, and run it a t a moderately fast idle, maybe 1,000-1500 RPM for 15 minutes or so. DO NOT DRIVE THE TRUCK! The crankshaft and rods will splash the oil/Diesel mix around like an agitator in a washing machine, and you'll get a better cleanup of the internal parts than you'd ever be able to do with a brush and solvent. Shut down and drain the oil while the engine is hot. If sludge and varnish has the rings stuck, It's possible this procedure will clean up the deposits and let the rings get a better seat. If not, you haven't spent much time or money trying to do a simple fix. Several people here have tried this procedure with pretty good results.
Jerry
