My friends - the best way to replace oil is to follow the either the manufacturer's guidelines or the oil company guidelines. Transmission oil(including automatic tran fluid -ATF) are generally very long drain oil and could last life long e.g. Mazda 3(as I quote from their manual) or it could be upto 130000 Kms. You need to check up the manual for their recommendation. Do not be fooled by the mechanic showing color, smell or taste of the oil and recommending a change. Believe me the mechanics in India and Canada all do those acts.

Having sold all kind of oils for Esso/Caltex - HP in India, I have experience this first hand. I used to challenge the mechanics on their advise if they could recommend oil change based on color which is very widely practiced. It is all BS and technically not possible.

Just to add even the oil change originally started with 5000 miles recommendation which is equal to 8000 kms but most of the mechanics in India and Canada talk about 5000 kms change. Also most of the new cars come with auto recommendation electronic system so follow that, but do set your oil counter to 100% once you have changed oil for the system to correctly calibrate the life of oil.

Hope this helps!

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Ash