I agree with Jim...85w-90 GL4 gear oil.........
The synthetic is super slippery and is likely the cause of the shifting issue.

The synchro rings are brass and are probably not liking the synthetic oil...

The synchro rings need to be able to grab the cone on the gear they are mated with and the friction between the two parts causes then to match speed....

I would get the box warm (Drive the rig 10 miles or so) Dump the synth and let it drain really good......Refill with the regular Dyno 85W-90 Gear oil.....

My bet is your shifty issue will go away.....

Synthetic oils are not always a good thing......Frictional quality is at times not the same and then parts that are supposed to drag a little (Synchros) do not do their job....

Also it is possible that the tranny internals are getting tired....And the super slippery oil was enough to bring things to light.....Try the regular oil....