15w40 in mine.
The same GMC manual calls for a minimum of 25 psi at 2750 rpm warm on a used engine and 40 psi on a new/rebuilt engine.
My measurements were taken using a 1% accuracy 270 degree sweep 4" diameter gauge.
Last edited by Bill Hanlon; 09/20/2025 3:45 PM.
'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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