DOT 4 vs DOT 4+ brake fluid -

[pcn1]

Just did the brake fluid change on my old Mercedes 1998 SL320.

Reservoir cap says use DOT 4, which is handy as all my other vehicles use the same.

After I finished I checked the cap again and noticed it said DOT 4+...... bugger, whats that I thought?

A quick google says not to worry, its fine to use DOT 4 but the + version has a higher boiling point for better performance at higher temps.

Me being me, I'll bleed it out again to the correct fluid, but in the mean time what do you guys think ?

Cheers

[mmm-five]

Check the wet & dry boiling points of the DOT4 stuff and see if it's better/worse than the DOT4+ stuff you're planning to use.

In a lot of cases, DOT4 stuff will exceed the minimum specs for DOT4 (and DOT5 in many cases).

If your current DOT4 fluid exceeds the minimum for DOT4+, then leave it in.

[pcn1]

I guess for that I would need to know the specs for DOT 4+ in 1998, and compare to DOT 4 of 2023 !

[mmm-five]

pcn1 said:

I guess for that I would need to know the specs for DOT 4+ in 1998, and compare to DOT 4 of 2023 !

I don't believe the specs changed over time...they just introduced another spec when they wanted higher performance specs.

I was just saying that you can check the specs on the DOT4+ bottle and compare them to those on the DOT4 stuff you've already got in there.

Assuming Mercedes are not using 'DOT4+' purely as a marketing term to ensure customers buy the stuff from the dealer at a huge markup, then the designation doesn't change the wet/dry boiling point, but rather changes the cold weather (-40ºc) viscosity...which states ≤1800mm²/s for DOT4 and ≤750mm²/s for DOT4+.

I don't know which is better...and DOT5.1 is somewhere inbetween at ≤900mm²/s.

[richhead]

the only thing that will ever be a problem in the real world is if the differance will attack the rubber parts of the brakes, and in this case no it wont, so dont loose any sleep

[donkmeister]

mmm-five said:

Assuming Mercedes are not using 'DOT4+' purely as a marketing term to ensure customers buy the stuff from the dealer at a huge markup, then the designation doesn't change the wet/dry boiling point, but rather changes the cold weather (-40ºc) viscosity...which states ?1800mm²/s for DOT4 and ?750mm²/s for DOT4+.

I don't know which is better...and DOT5.1 is somewhere inbetween at ?900mm²/s.

These units are for kinematic viscosity, which is (essentially , in simple terms) how easily the fluid will flow (rather than how easily something can move through the fluid, which is called dynamic viscosity).

In these units, smaller numbers mean runnier fluids. So, assuming the DOT4 and DOT4+ were at the absolute minimum of spec, the 4+ would be a more efficient hydraulic fluid at very cold temperatures due to being able to carry momentum through the hydraulic system with less resistance. In simple terms, at cold temperatures the 4+ would give you a normal brake feel when the 4 would give you a heavy sticky pedal.