A little historical perspective: SAE 50 engine oil is standard for spicer trannies year round and specified for clark and warner below 32F. 140 gear lube above 32F. Fullers are to use gear lube of varying viscosities (depending on temperature) only, never engine oil. The GMC service manual states that SAE 50 is to be used in the clark 5 speeds and can also be used in the 4 speeds (normal lubricant for the SM420 is 90 gear lube). SAE 50 is equivalent in viscosity to 90 gear lube.
