Condensed answer: Most of SRAM’s hydraulic brakes rely on DOT fluid and therefore cannot accept the mineral oil of Shimano. There’s also a SRAM brake line (DB8) that operates with mineral oil. However, in that case, it’s still not advisable to use mineral oil from another brand as the warranty will be lost.
Hydraulic disc brakes control the movement of the pistons and brake pads via brake fluid running in the brake cables.
When the rider squeezes the brake lever, the pressure increases and the fluid pushes the brake pads towards the brake rotor.
When the rider releases the lever, the pressure decreases. Then, the fluid “backs off” and frees the rotor.
Mineral Oil and DOT Fluid
The two most common types of bicycle brake fluids are mineral oil and DOT fluid.
DOT fluid is the most used brake fluid because it also appears in the automotive industry. DOT fluid undergoes strict regulations by the Society of Automotive Engineers and the Department of Transportation. (Note: DOT means Department of Transportation).
Mineral oil, on the other hand, is not regulated. Hence why all producers have a proprietary formula kept in high secrecy. For that reason, it’s recommended to use the oil that the brake manufacturer suggests.
That said, at the end of the day, mineral oil is still mineral oil regardless of the brand and can often be used even when the brakes’ manufacturer is different from the mineral oil producer.
However, it’s absolutely crucial to NEVER use DOT fluid instead of mineral oil and vice versa to avoid brake failure.
In other words, always use mineral oil for mineral oil brakes and DOT fluid for DOT fluid brakes.
As explained, most SRAM brakes rely on DOT fluid and therefore Shimano’s mineral oil is completely incompatible with them.
Is it ok to mix mineral oils from different brands?
The standard answer is no for the following reasons:
- You will lose the warranty on the brakes as you are not following the instructions of the manufacturer.
- There are no standards for mineral oil. Consequently, producers rely on slightly different formulas. One cannot know with 100% certainty how the substances in each formula will react when exposed to those from another brand.
- The seals found in the calipers of hydraulic brakes are designed and tested to operate with the specific fluid of the brake manufacturer. Thus, there’s a chance that the seals will experience damage when mineral oil from another brand is used.
Here’s the official stance of SRAM on using mineral oil from other brands for their DB8 mineral oil brakes:
No, we have not tested all other mineral oils, and cannot guarantee performance or safety. Damage resulting from the use of brake fluids other than Maxima Mineral Brake Oil is not covered under warranty. (source)
FAQ: What are the advantages and disadvantages of hydraulic brakes in comparison to mechanical units?
Hydraulic disc brakes have the following advantages:
- Self-centering (you don’t have to play with the pads to prevent rubbing)
- Stronger “squeeze” for the same effort (Hydraulic brakes make it possible to stop with 1-2 fingers because the levers do not have to be squeezed as hard.)
The downside of hydraulic disc brakes is that they require more sophisticated maintenance (bleeding) and cannot be repaired in the wild. Once the fluid is lost, the brake no longer works. Hence why people who want a simple setup that just works often stick with mechanical models.
Summary: What You Need To Know
- SRAM hydraulic disc brakes use DOT fluid except for one model (DB8) which uses mineral oil. Shimano’s brakes use mineral oil.
- DOT fluid and mineral oil should not be mixed. Use DOT fluid for DOT fluid brakes and mineral oil for mineral oil brakes. Otherwise, your risk damage to the seals of the brakes which will lead to complete brake failure.
- Even though SRAM’s DB8 brakes use mineral oil, it’s still recommended to stay with the manufacturer’s recommendation for mineral oil to avoid losing the warranty of the brakes.