Just affected repair on saab 1.9 16v which also uses the same gearbox.
In the gearbox turret there is a fixed pin for a pivot point that goes through a nylon bush, moisture gets inside this and corrosion between bush and bracket make it tighter and tighter on pin. You can lubricate and it may help for a little while but will not solve.
You can buy a kit (£45) that gives you everything to do job (there is also a cheaper version £20 something but don't know if componenets as good and doesn't come with correct drill to remove old pin).
From start to finish process took me 1hr (bout 15-20 on repair itself) and can confirm it's transformed gear change from gorilla strength required to better than on my 156 20v (which will also get done as soon as it starts to get tightish). Nobody bothers with exotic oil in saab GM f40 gearboxes any more. The repair kit comes with everything, drill thing to take out old, pin to hammer old pin through, cleaning material to fit on same pin and clean up corrosion in hole, new metal bush (not nylon like old), new bolt on pin...
Highly recommend it to anyone with an f40 box and stiff change or stick that no longer centres.
first pile of results here (2 stroke is tried and tested one with lifetime warranty) but others may be just fine) http://www.ebay.co.uk/sch/i.html?_od...epair&_sacat=0
Hope you enjoy (guess this may also be useful to 159 guys as age creeps up)