AP Racing Forumla DOT 5.1 brake fluid
 

I have 3 500ml liter bottles of AP Racing Forumla DOT 5.1 brake fluid laying around. It is supposed to be very good. It came with my BBK Stoptech kit for my 300ZX. I now thinking of using Motul RBF 600 full synthetic for that and the AP on the X5.

Is this fluid OK for use in the X5 OEM brake system?

It says it "mixes safely with DOT 3, DOT 4, DOT 5.1 fluids" on the back of the container.

It should be fine. Once you bleed the brakes, most of the fluid will be the AP stuff but there will still be some old fluid left in the ABS system. IIRC only the dealership has the tools to properly flush that without introducing air..

even if using motiver power bleeder?

Several times a month, for the 911 track car. Used it twice a year on the old Audi too. Great piece of equipment.

I use either Motul 600 or Brembo Racing fluid in the 911 and it takes a lot to get either to boil at the track. But the Motul is a DRY only fluid- intended for racing. Once any moisture gets into the system (natural humidity, or boil the fluid) the wet boiling point is pretty low. Pedal will feel spongy. Not sure I'd want to run it on the street without flushing it once or twice a year.

Looks like the AP has a pretty high wet boiling point, so that should work OK on the street.

Thanks for your input :)

Am I better off just using DOT 3?

DOT 4 is rec'd for X5...if you aren't going the unecessary
5.1 route, imo.
BR,mD

The fluid doesn't matter, but DOT 5 must be changed ever year.

Agree with motordavid- have to use Dot 4 or Dot 5.1 (NOT Dot 5) in the X5. Dot 3 has a lower boiling point.

Dry BP Wet BP
Dot 3 401 284
Dot 4 446 311
Dot 5.1 518 375

Do NOT use Dot 5- that's a silicone based fluid that is typically used only in collector cars and some millitary vehicles. In a car, you'll almost always have a spongy pedal.

Why not Dot 3? Look at the Boiling Point specs. The dry BP is 45 degrees lower than Dot 4. And the wet BP is 27 degrees lower. Keep in mind that the moment you open the brake fluid can and put it in your car, you've introduced moisture into the system, and since brake fluid is hydroscopic, it will absorb moisture sitting in the brake fluid reservoir. That's why you have to change your brake fluid every two years. The difference between dry and wet is a significant difference in temp capability.

I run Motul 600 in my 911 track car which has a 600 degree Dry BP and is a Dot 5.1 fluid. I don't worry about the wet point because I flush the fluid before each track event. If I ran a normal Dot 4 with a 446 Dry, I'd boil the fluid in 20 minutes.

In my X5, I run ATE Super Blue which is high performance Dot 4 synthetic fluid, with a 536 Dry BP and 396 wet BP as I recall. OE fluid in many German performance sedans like older AMGs, and is actually available at Benz dealers. I like the extra margin of safety between the 446 of a normal Dot 4 and the ATE. Just like in motor oils, not all brake fluids are equal either.

Regardless, leave the Dot 3 for the nice elderly lady down the street driving the 88 Chrysler.