Inadvertently got in an argument...

Jhill

Too many engineers think because they are an engineer in one thing that they know all about everything. The good engineers know what they kno and are willing to learn what they don’t. For the former engineer they will be the first one to tell you they are an engineer to which one of the best responses is to say really? What kind of train do you drive?

You are correct though DOT 3/4/5.1 are all glycol based DOT 5 is silicon based and not compatible as you know. And for boiling point concerns 5.1 can go in 3 or 4 but the reverse is not true, not that it will instantly fail it just a boiling point concern. Then there gets the really technical as 5.1 viscosity is thinner (probably not noticeable by foot but by a brake lever it is) and then you have higher boiling point racing DOT 4’s that can have even higher boiling points than 5.1. But for your argument sake you are correct.