DOT 5.1 is not silicon based, it is a higher boiling point meant for DOT3/DOT4 brake systems. DOT5 is silicon based and cannot be mixed. The silicon in it can affect some of the rubber parts and may not have the correct addetives to condition the rubber seals on the piston and MC.
Here is a tip while bleeding. Run your bleeder hose up through your springs. Open the bleeder. Now air/fluid may come out, but bubbles will go to the top, and if you release the brake pedal while bleeding, it can suck in only fluid. This way you can bleed your brakes yourself.
Attach the bleeder hose to the valve, run the hose up through the spring, and down into something. Release the bleeder. Now fluid may come out, but air will not be introduced back into the system. You can now walk to your brake pedal yourself and pump it about 15 times (keep adding fluid to the master cylinder, you don't want to let the fluid level go too low because it could introduce air into the system on that side.) I've done this many times, and now you can bleed the car yourself. It can also save time because you don't open the bleeder multiple times.
DOT4 fluid obsoletes DOT3, its a higher boiling point, so DOT4 fluid is fine. The plus is that you can get stuff like DOT4 Valvoline Syntec for 7 bucks a quart at a local Autozone, so its very effective and its cheap. There are 4 and 5.1 fluids for racing, but some get upto 100$ a pint, and for the extra few degrees, its not worth it for street driving, thats more a race only thing.
