All gearboxes made from 2002 use a very specific gearbox oil called MTF0063 and that's all that you should use as far as I'm aware. It's bright red (looks a bit like strawberry juice). It does not mix well with other oils so great care is needed if you want to change it to something else.

 

Never ever put engine oil into any modern gearbox (or diff for that matter too)! Otherwise you'll end up with a very expensive repair bill after not too long...

Here is a reasonable description of the difference:
http://www.wisegeek.com/what-is-gear-oil.htm

 

If it is a 6-spd manual gearbox then after you empty the old oil you should fill it with 2.4 lt of fresh oil. Not more than that.
This is what WIS 2010 advises.