I did notice this as well, only not to quite the extent you might make it out to be. My old C50 in my '87 is original and at 143K shifts like it is new. In fact, I put Redline in it about 23K and 3 years ago, and along with new grease on the shifter pivot points and inline skate bearings replacing the rubber bushings on the shifter select points I just did a month ago, it's absolutely smooth as glass on the maiden drive.
My '89 SC with 37K has had Redline MT-90 in it since about 28K or so, not long after I bought it in '02. It shifts better after these few years, but it's still not as nice and fluid as the C-series. I feel it catching and fighting me a lot more to select the gear, but it's not hard to push or pull in once the gear has started to engage. At first I blamed it on the previous owner, but I think that's just the way the E-series is. It's obviously not seen much in the way of mileage, and I shift without grinding, so it should be as close to original as most people would see.
I'd try a good gear lube first off and see if there's a change. If not, that may be about par for the course.