It's a perennial mystery to me that almost every person in the world understands the need to change a car's engine oil from time to time, yet when I tell someone that I recommend changing almost any other fluid, I get a blank stare (or its telephonic equivalent). The blank stare is sometimes followed by a comment, such as "Well, I never changed that before!"
To me, it stands to reason that all automotive fluids have finite life spans. A couple of factors play into why a lot of drivers, especially older drivers, "never changed that before."
First, modern cars run much,much longer than ever before. When I started working on cars in the late 1970s (!), most cars were junked by about 100,000 miles. So if, say, the gear oil wore out at 60,000 miles, did it make sense to change it if the car was going to be scrapped in a couple of years - or less? In many cases, no. In fact, odometers in those days usually only had five digits! Today, though, almost any decent car will easily go over 250,000 miles without any catastrophic failures of major components.
Another big factor is heat. Today's cars run much hotter than the old dinosaurs did. One of the reasons for this is efficiency. When engines are run cold, they use more fuel. Computerized engine management systems allow engines to run more efficiently without a meltdown. Another contributor is the compactness of modern cars. Everything is packed tighter, so lots of stuff doesn't have much air space around it to help keep it cool. Heat accelerates the rate at which fluids deteriorate.
A third factor is complexity. Many modern systems are much more sensitive to fluid condition than they used to be. An example is brake fluid. Modern cars have anti-lock braking systems that have little passages and valves in them that don't like gummy, gritty fluid.
The last factor I'll mention also relates to modern automotive technology. Cars today are made out of many more kinds of materials than old cars. Different kinds of plastics, aluminum, steel, cast iron, copper, rubber, etcetera, go into cars today. All those different materials place more demands on the fluids that come into contact with them.
When I feel ambitious, I'll write about individual fluids, what happens to them over time, and what that means to you.
Thanks for your time!