In times gone by.... like from the 1900’s up until around 2000... a gearbox such as a rear axle differential in a car/PU-truck was never changed in the life of the vehicle.... which was probably in the order of 100K miles or so for the average person. (My ‘64 Dodge 440 inherited from my parents had 167K when I sold it but I’m unusual... I keep vehicles until they’re wrecked.... the only reason I sold it was the guy who bought it was needful and couldn’t afford anything. I sold it (and it’s leaky, push-button transmission) for $50 in 1976. But 30+ years is the norm for me.)
Anyway, closed helical gearboxes were never changed and manual transmissions were rarely changed until 50K or so. THEN along came the millenium and all of a sudden car makers put limits on rear differentials on the order of 30K miles for oil changes and often specify SYNTHETIC on top of that! I ignore that and do it at 100K using whatever fluid they call for , but I am NOT going to do it every 30K miles.
Bush-Hog/shredder/etc?? Never. Use ordinary GL5 80/90 gear oil and check the level occasionally. That’s it.
BTW, in case anyone wants to know if I have any basis for my actions.... I paid my way thru college and flight school rebuilding differentials as a rear-axle-specialist for Gulf States Toyota in the early ‘70s, driving to/from work in my ‘51 Chevy P/U bought from a gov’t (US Dept of Agriculture) auction for $150. Six cylinder, manual tranny, with a heater and a steel-floor bed... and that’s ALL it had. Friends said the color matched my personality.... olive drab.
