So question then: We rode roughly 50 miles from Furnace Creek in Death Valley to Scotty's castle, then back. This was all at about 55-60 mph. Would you consider this riding for "long times"? I just ordered a few oil filters and a case of Castrol Power RS 10w-50 oil (was cheaper to buy a case of 6 on Amazon) and have no problem changing the oil more often if that helps.
I got the 2004 WR450F - It's liquid cooled - I believe the 99 model is fan only , hence the problems over heating.
I don't know man... The TE is a race engine basically. I know there are other 250s that can sustain long road travel, but their intended use was more oriented towards actual dual sport rides which involves more road riding - and less potent of an engine as a result.
Do you know how high you kept the revs? As I mentioned - from factory - there is a throttle limiter (not the redline) that prevents you from exceeding 6500rpms , and I believe gives you a 6 month warranty. When you power it up and remove throttle stop, among other things, your warranty is void. There is a reason for that


That said, I have a 2010 TE 250 - and absolutely love it in the woods / tight stuff.
I don't know how much more powerful a 310 is then a 250. But a TE449 , I would imagine, be fine for 55mph travel as long as you are weary of the oil and such. I guess it also depends on Sprocket sizes too. Problem with the smaller bores (250 and 310) is that they hold so little oil - and on some models, the oil gets sucked up through the airbox...
On my 2010 TE 250 - 6th gear is practically useless and garners no power. Maybe it's different on a 310. If you are sustaining 6000rpms - I wouldn't worry too much - just change that oil and maintain it well.