Mike,
My understanding is that a "manual" transmission doesn't generate heat in the oil like the fluid does in an automatic transmission does.
Manual's have gears and the oil just to lubricate the gears. Whereas automatics by there very nature transfer the engine power to the drive train using transmission fluid. The more load there is the more the transmission fluid heats up.
As far as not using 6th gear, you are going to get a thousand different opinions on that. Personally, I would use it on flat terrain as long as the engine can handle the load. Monitor the RPM and let us know what you end up doing. With gas over $3.00 a gallon, I think it is worth a try...
Keith
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