I have heard this many times over the years, but was never clear as to why. Now I know.
This is from an 8 speed Spicer transmission from one of our International flatbeds- approx 400K on the odo and it would no longer stay in 5th or 6th gear. Our mechanics dig into it, and now we know why!
I wish I’d snapped a pic of the new part, but both sides of that channel have been worn away by the synchros, and one look by the guy at the parts counter and he told us, “Tell your driver to keep his hand off the shifter when he’s going down the highway.”