Hi John.....I`m not going to be able to give you much in the way of first-hand info. as I`ve had no trouble with my gearbox to date. However, tranny`s are known to be a Hyundai weakness.
I would suggest you change the gear oil with a fully synthetic 75/90 like Millers CRX or Redline M90, preferably with Slick 50 gearbox treatment added.
Secondly check all the bearings & linkages for alignment, then spray with Waxoyl.
Lastly, double-check that the clutch-pedal is not grabbing too low down in it`s travel & is releasing properly.
(One thing you may experience which is not really a fault, is difficulty in engaging reverse......Hyundai themselves say that if this occurs, to try putting the lever into first gear to align the internals, before engaging reverse!.
Hope some of this helps, Nick.
