How often should you change the gear oil in the tranny and rear end?
When it's dirty... sounds like a smart aleck answer but I am serious.
Changed the rear and trans fluid on my '65 C10 because it was "new to me" and I had to change the pinion and an output shaft lip seal.
My wife's '07 Jeep Wrangler has over 100,000 miles. We bought it new and I have changed the rear differential gear lube about every 50,000 miles. The front still has the factory OEM fluids.
It really depends on what duty they see: light, medium or heavy duty use.
My hi-tech method: Unscrew the fill plug, stick your finger in the fluid. If it's coal black, change it. If it isn't and still looks similar to new GL-4 gear lube, let it ride.
