BMW Brake Fluid

Brake Fluid for BMW Street & Track

Brake fluid used to be simple.... DOT3 and then DOT4 for all cars. But then in the mid-2000s it got a little confusing. Our BMW brake fluid recommendations appear up front and some deeper background further down the page.  


>>> #1 Rule: Change Your Brake Fluid Often! <<<  


Brake Fluid Recommendations:  

1965-2003 All BMW Models  
Street = any DOT 3 or DOT 4 Normal viscosity  
Track = Super DOT 4 Normal viscosity, or DOT 5  


2004-2006 All BMW Models  
BMW began transitioning to Low Viscosity fluid during this time along with the switch from ASC+T to DSC traction control. DSC was a different approach to traction and stability control and the fluid needed to be thinner to ensure the system worked in extreme cold. The viscosity difference is about 50% thinner than the older Normal viscosity at extreme cold temps. If your car has DSC you should use the Low viscosity fluid for street use but you can use either one for track use. DSC appeared halfway through 2003 but not all cars got it at once. If you have a DSC button on the dash or center console then you have DSC and should use the Low Viscosity fluid.  
Street = DOT 4 Low or Normal viscosity, depending on DSC equipment.  
Track = Super DOT 4 Normal viscosity, DOT 5, or DOT 5.1  


2006+ All BMW Models  
Street = DOT 4 Low viscosity, DOT 5.1, DOT 6 (Class 6), or DOT 7 (Class 7)  
Track = Super DOT 4 Normal viscosity, DOT 5, or DOT 5.1

Low viscosity for street cars. For strictly track use it's OK to use a Normal viscosity fluid as it will thin out to functional levels. For dual-purpose street-track cars you can use either so choose the fluid that has the specs that match your needs unless you plan to be driving in below freezing temps.  


Brake Fluid DOT Ratings  

The US DOT is way behind the curve on ratings and standards for brake fluid. And this leads to a lot of confusion as technology and fluid demands have changed and/or evolved. Traditionally, brake fluid is classified based on boiling points and viscosities but it failed to address variations that muddy the waters. And further failed to address alternative materials such as silicone. This has led to a bit of a free-for-all in the brake fluid market. Adding further confusion, in 2022 the US DOT published Class ratings based on an ISO program. Maybe it's for the better if the US DOT takes a back seat to the ISO from now on.

Minimum Dry Boiling Point = boiling point of fluid straight out of the container  
Minimum Wet Boiling Point = boiling point at 3.7% water content (roughly equal to one year of use)  
Hygroscopic = absorbs moisture. Water is absorbed and mixed with the brake fluid.  
Hydrophobic = repels moisture. Water remains separated from the brake fluid.  
 

(1) - the DOT does not officially recognize "Super DOT 4". This is an industry label applied to high performance DOT 4 fluid that surpasses DOT 5 boiling points. See more below.  
(2) - DOT 5 was previously only referenced with the original Castrol SRF but it applies to any silicone-based fluid meeting the specs. See more below.  
(3) - the DOT does not make a distinction between "DOT 5" and "DOT 5.1" except where color and labeling is concerned. Anything with a DOT 5 rating needs to be labeled as silicone or non-silicone base.  
(4) - it seems that the US DOT skipped an official DOT 6 grade. But some manufacturers are adopting the spec on their own and claiming “Dot 6”. But without a true published standard it's impossible to know what it really is. The DOT has published a “Class 6” and “Class 7” spec which is the same boiling points as previous standards but with even thinner viscosities.  


Super DOT 4 vs DOT 5  

One of the misconceptions in the DOT ratings is that boiling point is the only difference from one to another. In fact, it's one of two primary tests. The other is the viscosity of the fluid, measured at two temperatures - minus 40°C and 100°C.  

All DOT 4 fluid is thick (normal) viscosity at no more than 1,800 mm2/s (-40°C) with boiling points of 230°C/155°C (dry/wet).  
All DOT 5 fluid is thin (low) viscosity at no more than 900mm2/s (-40°C) with boiling points of 260°C/180°C.  

There is no classification for a normal viscosity fluid that exceeds the boiling point of DOT 5, resulting in some confusion. Manufacturers of racing or high-performance brake fluid want to distinguish their high output DOT 4 products from the regular DOT 4 you find in a big box store. They are not allowed to call them DOT 5 because of the thicker viscosity so they came up with "Super DOT 4". These are fluids that exceed the higher temps set in DOT 5 but retain the thicker viscosity. These are an excellent fluid for E30, E36, and E46 track cars. E9X and later track cars should use a thinner viscosity DOT 5 or 5.1 fluid due to the DSC system requiring a thinner viscosity. The primary difference between a Super DOT 4 and a DOT 5.1 is the viscosity and they will have similar boiling points.  

Likewise, there is no classification for a low viscosity fluid that stays within DOT 4 boiling points. By temp alone these fluids are DOT 4 but are the thinner viscosity. The industry has stepped in to label these "DOT 4+". In Europe this would be the equivalent of a ISO Class 6 fluid so you will sometimes see "6" in the labeling or marketing (Ate SL.6 for example).  


DOT 5 vs DOT 5.1 (and Old vs New Castrol SRF)  

When the DOT 5 standard was released in the early 2000s the only product on the market that met the standard was the original Castrol SRF - a non-glycol silicone-based fluid. SRF was the only product for so long that "DOT 5" became synonymous with silicone-based fluid. When glycol fluid advanced to exceed DOT 5 boiling points (but remain low viscosity) the DOT was forced to release a DOT 5.1 label to differentiate between silicone and glycol. All DOT 5 fluid is low viscosity.  

DOT 5 silicone-based fluid is an outstanding racing brake fluid because it does not absorb water, therefore its wet boiling point remains extremely high. However, since the water is no longer absorbed it will boil on its own and turn into water vapor or steam. The vapor can produce a soft pedal or block the system completely. It's not uncommon for cars that are stored for long periods to use DOT 5 fluid because the pedal will still be firm even after sitting. However, silicone fluids need to be bled more often than glycol fluids to vent trapped air and purge water so it does not cause rust and corrosion. When switching from glycol to DOT 5 fluid it's recommended to run several flushes of silicone through the system to remove all of the previous glycol.  

At some point Castrol changed the SRF formula to reduce the silicone content and use a DOT 4 rating. This made it more user-friendly and the current "React SRF" is now miscible with other DOT 3 and DOT 4 fluids, meaning you no longer have to completely flush the system clear of old fluid. However, the React SRF still likely has a large silicone content which gives it the outstanding wet boiling point and performance that racers are looking for. Castrol recommends it still be flushed often and not left in service for more than 18 months.

Low Viscosity Fluid

BMW began using a thinner (low viscosity) brake fluid in the early 2000s when DSC was introduced. DSC pumps fluid at very high pressures in a very quick time in order for the system to react as expected. To ensure the system functions in extreme cold environments the viscosity had to be thinner than the brake fluid that existed at the time. BMW - and other manufacturers - spec'd a viscosity at cold temps that is about half what the typical fluid was. The viscosity tests are done at -40°C and 100°C. Unfortunately, there is no lab test required for a “normal” driving temp but based on lab testing alone the thinner fluid is required only for the extreme cold temps used for the tests. In normal driving (in the lower 48 states) where temp ranges rarely get below freezing you “can” use either low or normal viscosity although it's probably best to stick with manufacturer's recommendation.

For track use both viscosities thin to about the same viscosity. When you see the viscosity at 100°C (table below) most are pretty close, with the thicker fluids being more race-oriented. The lab test and certifications are only done up to 100°C so we don't have the data on what the viscosity is beyond 100°C or just below boiling. But in our experience, and after consulting experts, there is no need to retain a thin viscosity fluid when on track and you're free to use whichever best suits your needs.  


Hygroscopic vs Hydrophobic (Glycol vs Silicone)  

Hygroscopic fluid will absorb moisture. In a nearly-sealed system like your brakes this means that any moisture that develops will be absorbed into the brake fluid. Note that this will lower your boiling points, which is why it is so critical to change your brake fluid often if you expect to have optimum performance. Cars that sit for extended periods, especially outdoors, will develop water in the system and require a brake fluid change before track use. Water and vapor in the brake fluid will lower the boiling point but will also give you a softer pedal as the water will compress more than hydraulic fluid. Water can also freeze in brake lines and cause corrosion. Glycol is hygroscopic and is the typical formula found in DOT 3, DOT 4, and DOT 5.1 fluids.  

Hydrophobic fluid will repel moisture. Water and hydrophobic fluid will not mix at all. This is great for preserving the boiling points of brake fluid. You'll see silicone fluids with a very high dry boiling point and that the wet boiling point is not much lower. However, moisture is going to form or enter the brake system no matter what. If it's not absorbed into the fluid it will get pressed around the system until it settles somewhere, potentially causing rust and corrosion if it settles on iron components. Hydrophobic systems need to be flushed more often - not to refresh the fluid but to extract the water that resides inside. Currently, silicone is the formula found in DOT 5 (5.0) fluids.  


Brake Fluid Color  

According to federal guidelines, automotive fluids must be certain colors to be easily identifiable. All DOT brake fluid must be an amber color with the exception of silicone-based fluid which must be purple. Hydraulic oil or mineral oil used in non-brake hydraulic systems must be green (power steering, convertible top, etc). Engine oil is also amber and transmission oil is supposed to be red.  

Ate Super Blue racing brake fluid was blue in color because it was a "racing" fluid and not intended for road use. It had boiling points that exceeded DOT 4 ratings so Ate wanted to highlight this feature and make it stand apart from other DOT 4 fluids. The DOT took exception with this and forced Ate to discontinue sales in the US. The specs were the same as the Ate Type 200 fluid which is the correct amber color.  


Brake Fluid Advice  

Remember the dry boiling point is fresh fluid from the bottle. The wet boiling point is supposed to simulate one year of exposure during normal driving (fluid with 3.7% water). We wish there was a third data point of fresh fluid after a weekend at the track but there is not. If you change your fluid before every track event (required for most events anyway) the dry point is going to be your most valuable. If you drive hard on the street you should only look at the wet value and also the viscosity.   

Are you getting your brakes hot enough to need a higher boiling point? If you are, you may need to re-think your brake set up or investigate why. Boiling new brake fluid should be a warning flag that you have another issue, the brakes are under-sized, not getting enough cooling, or you need to change your driving habits. You may have moisture or vapor entering the system, lowering your effective boiling point or producing a soft pedal. Racing brake fluid is designed with a temperature cushion built in -- 600°F for new fluid is extremely high and it's likely other damage is taking place. Remember that factory OEM parts are not designed with regular track use in mind. Piston boots and seals will burn or break down at much lower temps. Even the most heavy-duty, professional-grade motorsport seals have a temp limit around 500°F. Pads transfer more heat to the caliper the more they wear (thinner = less insulation) and the fluid is the last link in the heat chain. Brake fluid boiling should be a "speed bump" before other serious damage or failure occurs. So ask yourself why you're boiling brake fluid in the first place.  


Brake Fluid Spec Comparison  
 

note: the data above changes without notice. We regularly update our pages but please note that manufacturer data may have changed without our knowledge.  


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