[Hairway_to_Steven]

I am curious: why would people use 4 over 5.1, when the latter is superior in the most important metric: boiling point?

 

[Hairway_to_Steven]

DOT 5.1 does not have a higher boiling point then quality DOT 4. Motul and Castrol have DOT 4 fluids with much higher boiling points both wet and dry then the recommended Motrex.

Forgive me - I thought we were talking about standards, and not specific instances of fluids that happen to conform to a particular standard. I could certainly see that not all DOT 4 fluids are created equal. So, to compare apples to apples, I went to Motul's site (this is a cut-and-paste):

MOTUL DOT 4 LV high wet boiling point (172°C / 342°F) is superior to conventional fluids DOT 3 (140°C / 284°F mini) and conventional DOT 4 (155°C / 311°F mini) and therefore enables longer product lifetime.

MOTUL DOT 5.1 high wet boiling point (185°C / 365°F) is superior than the requirements of the standards DOT 3 (140°C mini / 284°F mini), DOT 4 (155°C mini / 311°F mini) and DOT 5.1 (180°C mini / 356°F mini), and therefore enables longer use of this product.

Which brings me back to the original question, rephrased in light of this exchange: why would one run DOT 4 when just about every brand you would care to mention has a DOT 5.1 in their line that is superior in terms of the key metric? Perhaps the cost is very different?