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the thing is when most of these truck came off the assembly line they got MIL-L-2105 which
is a GL-4 see(MIL-L-2105 : Former specification equal to API GL-4 performance level).
read this it has all the info on gear oil you will ever need
http://www.steelsoldiers.com/index.php?name=PNphpBB2&file=viewtopic&t=8941&highlight=gear+oil