1- Gear oil should be changed as per mentioned in the manual. Normally after 40,000 Km, as extreme pressure additive it depletes causing wear of gear.
2- For engine oil API level mentioned on the can is critical. If you are using Fuchs Super GT 20W-50, it is API SL, which is good enough for 5000 Km. If you are using Fuchs Super Formula, it is API SF, good for 3000 Km. if you choosed Fuchs PCNG oil, if is API SJ, good for 3500-4000 km.
Fuchs is German brand and very good quality oil.