OK, if you guys really want something to chew on here.
Last week I needed diesel for the tractor & took a 5 gallon can to the local station. Without even asking, they wrote me up a refund ticket for the highway tax, and took the tax off the charge.
I asked and they said it was obviously for off-road use since I was getting it put in a can.
Next time, I might have to ask them, but clearly they will take the tax off.
However, the diesel was straight out of the ordinary on-road pump and was not red.
So at least in Oregon, the way to not pay the tax is to get it put in a 5 gallon can first...