Farmall 100 Stiff Shifter

I just changed the gear oil in the tranmsission on my Farmall 100. Now it is REALLY hard to get it into either 3rd. or 4th. gears. Once it is ingear, it runs and sounds fine. 1sts, 2nd. and reverse are smooth and easy. Any ideas??

Changing the oil wouldn't have caused that to happen. If anything it should have improved the situation, unless you used molasses instead of SAE90 gear oil.

3rd and 4th are on the same slider, so whatever the problem is it has to do with the 3/4 slider gear or the 3/4 shift fork.

Is it just sticky/gummy feeling, or does it grind?

Not gummy. No grinding.
I just have to push very hard on the shifter with both hands to get it to go into gear. It seems a bit easier to get into 4th. so if I get into 4th., I'll ram kit forward to get it into 3rd..
Doesn't matter if the tracor is running or not.
I'm thinking I'l have to remove the shifter cover and see if anything looks amiss. Won't be able to get to that for a few days though.

I'm leaning toward mkirsch's idea.

Only thought that I'd add is that the new oil could be having either a perfectly normal effect or exposing another issue that is tolerable but will ultimately require a repair.

One is that the new lube will obviously be slicker that the old, so you won't have as much friction on the input gear shaft to offset any friction generated by a dirty/sticky/or even completely clean pilot bearing/bushing. If the bearing/bushing is at all sticky, it will continue turning the driveshaft, even with the clutch pedal depressed.

The solution for that is to hold down the clutch pedal for a longer span of time before shifting, to give time for the input shaft to stop spinning. The input still spinning while the back-end gears are sitting still is what leads to what I call the classic Farmall crunch or grind when engaging in a gear.

The other side is that 3rd (not so fast) and fourth (really fast as these things go) gears spin faster than 1st and 2nd, so it takes a little more finesse to engage them. It has to do with the relative speeds of the powered vs. the driven gears are spinning. The difference in 3rd is noticably more that 1st or 2nd, but doesn't begin to compare with 4th (your "road" gear).

If you're not having a crunch or grind, it could bery well be what mkirsch suggested, in which case you'll wanna pull the cover/shifter off and clean things up and look for wear, especially on the 3rd gear side. Or for any crud that might be binding up the shifter on those gears.

I finally had a chance to look into this problem yesterday. Turns out that the bolts holding the rear shifter rail guide in place had loosened up about 5/16". Not exactly sure HOW, since there are metal tab locks to prevent that. Anyway, tightened everything down, relocked the tabs and everything is smooth as silk.