Brake Fluid Flush/Mix DOT 4/5/5.1?

[M3AN 4015]

So, like many of us I imagine, I have no idea what brake fluid is in my imported car (130). I'm assuming it's standard Dot 4.

I want to do do a flush/replace, I realise I can't used Dot 5 if the car already has Dot 3 or 4 but I understand Dot 5.1 is fine because it's backwards compatible with everything.

Given it's beneficial properties, even if I don't need them, is conventional wisdom to got with 5.1? Is cost the only disadvantage? $100 for a full flush every 5+ years doesn't concern me.

 

[Jacko 2077]

I run Castrol React Performance DOT 4, Which is essentially the same performance as DOT 5.1 ( and much higher than average DOT4 at 180/260c ), totally happy in BMWs and cheap as chups  https://msdspds.castrol.com/bpglis/FusionPDS.nsf/Files/B0F5626B0EB98D0B8025843C00213662/$File/BPXE-9VCL2X.pdf

Edited October 10, 2020 by Jacko

[M3AN 4015]

Yep, that's what I run in the M3 and no complaints... I wonder if the anti-hydroscopic properties of 5.1 are worth the cost?

[Eagle 1515]

Used to use the Castrol but its get real expensive if you do a few cars or didnt quite have enough. Penrite superdot has similar specs iirc  and is far better value in the 4.4L bottle esp when on special.

[HalfJobHarry 106]

If you keep up with the complete fluid replace every few years then any hydroscopic properties are likely not worth it. 

Interestingly BMW often specify the DOT 4 Low Viscosity fluid...which when I went to the dealer for it they initially told me "they don't sell it"...then when I produced a BMW part number they they were surprised saying "nobody never bought that before"

Apparently there is "need" to run the LV fluid with BMW ABS/Traction Control ...but frankly I'm certain garages the world over have been putting in regular DOT 4 forever without ill effect.