School me on brake fluid

[Bruce Parker (RIP)]

I chose regular DOT 3 hydraulic fluid for my brakes, clutch and accelerator systems. I recall my thinking was that because I would re-bleed/refresh every couple of years it would be fine, my worry being moisture buildup and therefore rust. I can't complain, except this scheduled maintenance when I took things apart I noticed black buildup on the cylinder bores of the clutch and accelerator slaves. The clutch is below the starter near the rear engine block (270 GMC) and the accelerator slave is under the exhaust manifold just forward of the exhaust pipe (there is a sheet metal heat baffle). Figuring the problem is heat, is there a better fluid to use? A higher heat DOT 3? What are the complications if I mix as there is still regular DOT 3 in the system?