Original GMC factory oil filters were of the "bypass" type. Not all of the oil (actually very little of it) was filtered each "pass" through the engine. The filter media was designed to catch very fine particles, much finer than a regular modern filter. The filter gets it input near the top from a tube (1/8" if I remember right) connected to a fitting on the front of the block. Filtered oil exits through a hose (made of unobtainium) from the bottom of the filter can and connects to the block just forward of the bottom of the distributor. Fram 51001 (NAPA 1001) filters fit, but in recent years some of the filters ship with improperly sized lid gaskets. I've reported this problem to Fram and got nowhere in getting it resolved. Two gaskets come with the filter. Mine needs the smaller of the two, but the gasket is just a little too small OD.
'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12
'52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.
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