Dears,
Assalam-o-Allaikum and guzashta Eid Mubarak 
guys i have mehran vxr 2006, i bought it at 40k mileage and now its going to be 80k in a few days. Now i've heard that gear oil should be changed in a car after 40k and i also DONOT know that it was previously changed or not. My car gears have recently become hard so am thinking of changing the gear oil. Kindly suggest in this regard, also I've heard from some person that changing gear oil usually create problems like some sounds etc. How much of this is true?
Looking forward for expert's opinions and suggestion of good gear oil!