Just changed my fluid at 45K miles, and the old stuff didn't look as good as the new, but I'm wondering what happens to it? It's not contaminated by blowby like motor oil, and it's not super heated or subjected to wearing bands like in an automatic. So what happens to it?

PS. Not finding metal chunks in your old fluid is a grrrreat feeling!

 

It still gets hot, so the heat up/cool down is probably what wears it down.

 

Well, it actually does break down over time due to the temperature cycling and the syncro materials breaking down and becoming suspended in the fluid; not to mention the actual gear material which breaks down over time and also becomes suspended in the fluid and stuck to the magnets inside the tranny.

So to answer your question, yes, it does age and needs changing from time to time.