I am no expert but as far as personal experience goes, I got my vitz 2009 CVT fluid changed almost 8 months ago. Its a daily runner doing 30-40 km daily and I have found no issue after the change. Actually it had no issue in transmission before the change either but i got it changed as saw some people recommending a change after 40k km of running.
I suppose Toyota Defence you mean the showroom located on Korangi road. I also found their advice confusing and unprofessional so never got anything done through them. They also suggested removal of Cat Converter. in my experience these Toyota authorized workshops generally try to sell you the services that you don't need (like injector cleaner every 5k km).
In the end I got the CVT oil changed by a small workshop suggested by a Pakwheels member. They got the job done in around Rs 1000 including new seal. They offered to wash the oil filter with petrol but I refused as I was not sure if the residue can harm the new CVT fluid.
The workshop was located near Kashmir road but I can't recall the name at the moment.
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