E63 Gearbox / Trans Fluid Change

[Rick62]

Hi all. Has anyone here changed or made a point of changing their Transmission fluid / gearbox oil in the 635d / E63. I was doing some "due diligence" before purchase and a local specialist said that that is one of the most important things to either check for or have done after purchase. Anywhere between 50k and before 100k was what he advised. Now I know that it is a sealed for life unit but so is the Range Rover and the recommendations are the same for that too! I know some will say "he would say that wouldn't he as he wants your money" but I have been looking around on the internet and , yes, it is a recommended thing to do, though not by BMW though!!
Am I looking for problems that really are not there or is this a viable preventative maintenance item? I know it is not cheap to have done but then a new gearbox would be even worse!!
Cheers for any advice that you can give.