oil leak from gear box gear lever inner shaft

Not sure where you buy part 040079 from. There are a number of potential suppliers. I can confirm NOC don't have a copy of the drawing. Perhaps Andover Norton have the drawing or a copy.

So what they send is what they think works, based on experience, heresay, or measuring an old o-ring.

If you can Duncan, measure the diameter of the indicater shaft at the bottom of the o-ring grooves. Measure the hole diameter through the gear change shaft. (Or indicater shaft diameter plus a couple of thou)

The difference is twice the gap for a squashed o-ring. Now decide for yourself if 2mm section or the (probably imperial) section supplied is the way to go.

When I did this exercise was in no doubt that the Lightweight o-ring grooves on this shaft were sized for a metric ring. I'm not alone in doing this exercise and coming to the same conclusion. We thought the imperial ring on offer was too small to assure a seal.

I'm sure you will be able to sort the problem. So annoying when they drip. 

Best wishes

Peter