One should keep track of the mileage it has undergone if you are the 1st owner..
If the car is bought second hand, the previous owner should be milked of all such relevant information for future references like transmission oil, belts and fluids etc.
If no such information is available then you have two options
- Don't fix it if it ain't broke
- Fix it before it gets broken at an ugly hour
If its me, i would change the part or the fluid that im in doubt about, because sooner or later you have to replace it, so why to stay preoccupied and worried, do it now.
Secondly the cost of replacement in time is much lesser than the cost of breakdown, financially as well as psychologically (and at times socially as well).
The mechanic (if a competent one) would advise basing on his experience or may be by observation, but at the end of the day, (if it breaks down) it would be you sitting in your car on a road curb and cursing the moment when you decided to take some more juice out of it before replacement.
So I suggest 'Take it for a change, before it takes you for a change'