How to find out if Transmission oil needs to be change ?
what are the precautions required.
what type of tranmission oil is better ?

all this will be done by machanic. just want to make sure he is not BS with me .

seriously, this has been asked like 5 times in the last month.

You can use the search function aswell but i'd say about every 2yrs or 40,000km for fluid and filter. Check the fluid to see if it looks good. Use the recomended fluid for your trans. I use amsoil synthetic but you can use which ever brand you prefer.

What year? You never said if this was conventional or CVT.