The crash box trans in your truck can use just about any 90 weight gear lube, since there are no synchronizers to get in trouble with the super slippery synthetic lubes. I'd suggest using a general purpose 90 weight hypoid lube in both the trans and differential. Later model transmissions like the SM-420 need a straight mineral oil without hypoid additives to keep from grinding gears on a downshift. The N-series Ford tractor specification mineral oil transmission lube sold by Tractor Supply is a good one to use in the 420, and hypoid gear lube in the rear end.
Jerry

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