Gearbox oil

Hi!
I own a Micra 1.3 from year 2000. My car has manual transmission, and has 50000kms on the clock. I would be intrested in how often do I have to change gearbox oil in my car. Some people say, there is no need to do that, the oil is good for the life of the vehicle... So if you can help me, please...
If I have to change the oil. Is Castrol SMX-S 75W85 fully synthetic good for my Micra?
I use Valvoline 10W40 DuraBlend for my engine I'm changing oil in engine after 10000kms. I've got 4 liters of Valvoline 5W40 SynPower in my garage.. Is it also OK?
Thanks in advance...

Personally I would only use 5W40 if the engine was being operated under extremely cold conditions.
10W40 is OK down to -20C

for manual gear boxes, you do not need to change the gear oil very often. Following are the instances where you need to change the gear oil;

1. If the oil is dis-cloloured
2. If you feel bit hard in shifting (if you loose smooth shifting)
3. If a gear humm is audible
4. If gears you experience gear crashes

these were told by Sri Lankan local agents for Nissan.

Thanks for all the help... I'm from Hungary, and the winter is coming.. Temperature never above 5 degrees. Not extremely cold.. :) I understang you recommend 10W40, but can I couse any possible damage or extra wear by using 5W40 for this winter?

And it was a big help with the gearbox too, I feel that gears change less smooth than before.. I think I will change the oil..