I've been using 90w GL-3 in my '52 GMC with the same SM-420 transmission as your '59 for 45,000+ miles. No telling what was in it when I got it. Autozone has it for $11/qt.
'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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