Originally Posted By: BrocLuno
15,000 miles and 17 years is nothing for manual transmission and dif oils. There are trucks running older oils than that in daily regular intermittent service ...
Yeah, sure I change my tranny fluid at regular intervals - 50,000 miles regular. Time is pretty irrelevant to these oils as the system is mostly sealed. Yes, there is a small breather cap, but it's like a 1/4" hole. They do get warm, but not really hot, so thermal breakdown is very slow unless you like to hit 100 mph in Death Valley regularly at noon ...
Gear oils are very stable. Some cars and trucks will die with the original oil in teh transmission and differential. You are not hurting a thing by putting this off until it gets to say 25,000 miles
When I was a kid, a long time ago, I worked at a car dealership as their oil change guy. In those days no one ever requested manual transmission or differential fluid changes. Just didn't happen. I asked our best mechanic about it and he said there was no reason to do it. I realize now that was probably not good advice, but we opened up plenty of transmissions and diffs. for something broken. But you did not see many worn out gears even with high mileage and original fluid. Guess that early experience colored my thinking about it.