How do the transmission and rear differential fluids turn brown or black? They go in almost clear and they are in a closed system. Whats the deal?

 

Parts are wearing out leaving little pieces of metal in the oil...

 

I've changed manual tranny fluid on a lot of cars and it was never black. There is a lot of shear and heat that goes on, though, and that would degrade the oil eventually.

 

heating and cooling/wear. when the Fluid's start to break down it cause's the color change.

 

Same reason motor oil turns black. The constant and intense high pressure mashing of the fluid between things like gear teeth and bearing races causes the fluid to break down and literally decompose into its original ingredients along with byproducts as a result of the decomposition. That just happens to result in a darker fluid than what you dumped in there.

This is the main advantage to synthetic lubricants; they are much tougher to break down.

 

Interesting, so could you use synthetic transmission fluid. Is it available?

 

Yes, of course. Mobil, Redline, Neo, I'm sure there are many more. Do a search.