With mineral oils, they have a finite lifetime and it's nowhere near as long as many people would like to think. Likewise for synthetic oils, while they last longer than their mineral counterparts, their lifespan isn't infinite.
Regardless of what oil is eventually decided on, it will still need changing regularly which will also give an indication of the gearbox health if there's any swarf, dirt etc in there that shouldn't be.
Similar goes for the back axle, PAS and so on.
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Cheers
Dave
Next Door to Top-Gun with a Honda CR-V & S Type Jag  Volvo gone but not forgotten........
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