Overdrive is 4th gear, period. If you go fast enough for the system to feel that 4th gear is appropriate, it will use it, but that shouldn't happen short of a fairly smooth dirt or gravel road. Turning O/D off is probably a good idea in those conditions though.
As for manually selecting a gear, that's usually wise. I'd rather be in a lower gear (2 or 1) on the trail at low speed and take advantage of readily available engine braking. Keep the RPMs under 2500 or so for the most part, shift up if you spend much time at 2000 or better unless you're engine braking.
Brent
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