Hello from Mr. Offtopic. DOT 5 is dead in the water, apparently ABS systems could cause aeration and thus made it unusable in cars (as per wiki). That's why DOT 5.1 was made as it has the temperature ratings of DOT5 while still being ethylene-glycol based. The video is still up, brake fluid part starts at 3:25:
https://www.youtube.com/watch?v=oF2vAOmplUI As for why DOT, as has been mentioned, DOT 5.1 is DOT 5.1. The DOT in the name stands for Department Of Transportation, meaning it's tightly regulated. That's why DOT fluids from different manufacturers can be mixed and matched. So you can...