WD-40 is a good start and will have prevented some of the worse case corrosion but is not really heavy enough to last long or provide a lot of lube.
Toward the bottom of the steering column and an inch or so above the upper lever there is a small hole on the side of the shift tube that needs a few drops of oil. Oil will run down inside the tube and will leak out a slot the tongue pin to the select rod slides in to take care of the splines the shift tongue rides on. A bit more will continue down to lube the R-1 shift lever where it rotates on the shift tube. Don't get carried away with the oil in the hole as what doesn't go out the slot will continue to the end of the tube and out all over the steering box and floor. There is no provision made to lube the upper 2-3 shift lever so it needs a few drops of oil placed at the clip and shim washers at top of the lever so some can run down the tube and get in the space between the lever and tube. The two levers need to be absolutely free to independently rotate on the shift tube. If one is sticking when the other is selected to move, the two will try to turn as a unit and generally cause lockups or missed shifts since the interlock in the trans will prevent two gears at once.
Wear at the rod ends where bushings at pivot points need a snug fit has been found by many having issues and the linkage being out of adjustment can also cause lockup problems. I would suggest you read thru some of the service info on the 41-7 Clipper as the 50 mechanism is identical. There are half a dozen articles on sticking and lockup issues pertaining to the Clipper that can be accessed from the service index.
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