8 minutes ago, ABear said:The one main item to be concerned about is the boiling point, and each manufacturer must provide testing proof that their fluid meets that minimum boiling point for that DOT number. Exceeding the minimum boiling point is fine, but in the case of most antique vehicles you would have to be driving awful fast or doing a lot of driving down steep long, long mountain grades to which one would reach the boiling point of DOT3 or DOT4 fluids.
This!
If you take your car on the track, with repeat hard (read threshold) braking, boiling your brake fluid is a concern. That is why for track events they insist on new brake fluid every time, to make sure there is no water in the brake fluid which can boil. For collector cars, this is a non-issue unless your going down Pikes Peak riding your brakes all the way (however, with drum brakes, you most likely will lose your brakes from over heating before you boiled the brake fluid)!
Robert