I had this problem for a while ie gear change getting stiff and making a creaking sound when hot. Its an issue that has been widely reported on here, and although I did the recommended things (change gear oil, clean cable ends etc) it didnt solve it. However I recently got to the root of the problem so I am reporting it here in case others have the same issue.
The problem is a part called a Gear Selector Link lever, a triangular plastic part that goes between the cables and the gear selector unit. The long spindle (arrowed in my photo) fits into a sleeve on the top of the gearbox selector unit. This plastic spindle obviously degrades and swells and is unable to rotate in its sleeve correctly - this is where the stiffness and creaky noise occurs. Mine actually snapped when I replaced it and it was a bitch getting the stump of the spindle out. But once replaced and gears reset, its now a different car with gearchange as slick and smooth as it should be.
The part cost about £25 from ebay and is an easy fit (if it doesnt snap like mine did!) - it takes longer to remove the airbox to get to it! But there are different but similiar parts. First one I ordered did not fit despite seller assuring it would fit my motor (1.9 diesel). But br referencing part number stamped on my original broken bit it was easy to get the correct one, which I should have done in the first place!
Hope this info will be of use to others.