One of the key stones to reach a long lifetime of your engine is frequent and regular engine oil change. If you are hesitating about the cost of these frequent oil changes (especially with larger engines consuming large quantities of oil each time) I would like to recommend to use a good but cheap oil (with correct viscosity and proper quality ratings of course) and change frequently instead of buying the most expensive oil and change very rarely.
When changing the gearbox oil, you not only give the gearbox fresh oil, you will also have the opportunity to clean out all the small metal fragments that have been created inside the gearbox and collected on a small magnet on the drain plug.
The easiest is to simply follow the SI (Service Interval) lights indicating Oil Service or Inspection. When maintenance is to be done, the yellow LED will come on together with either the Oil Service or Inspection indicator. If the maintenance is not made, the red LED will also illuminate.