The manual says police, taxi, FREQUENT TOWING, change at 20,000. We aren't police or taxi but speaking for myself I certainly do more than frequent towing and I'm pulling 26,000 lb total through the rear gear set so 20k is what I do.
If I did less weight or less often I'd probably change less often. No matter how much or how little or how frequently or infrequently I'd at least do one change initially.
It's sort of like a small engine generator, the first and even the second oil change will sometimes look surprisingly metallic ish. After that my oil will start to darken at less than the 100 hour Honda recommendation but it has never again looked metallic. Im not saying the gear oil will look anywhere near metallic but the concept is the same. The gear set has to break in.
