Gear oil doesn't generally wear out, so probably not even necessary to change transmission or differential oil unless they've gotten water in them. My 55.2 service manual says it may be advisable to change if subject to "severe service" but that's not defined and you probably don't use your truck that hard. My modern diesel truck has over 250k miles on it and I have never changed the rear axle lube.


Kevin
1951 Chevy 3100 work truck
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1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car)
Busting rust since the mid-60's
If you're smart enough to take it apart, you darn well better be smart enough to put it back together.