Hydraulic Mineral Oil
Why mineral Oil
Mineral oil is a great alternative to DOT Brake fluid. Mineral oil is not caustic and is non-toxic, but the main performance advantage to the rider is that mineral oil will not absorb water from the air like DOT fluid will. This absorbed water causes gas bubbles to form in the system which creates a 'sprongy' feel.
It is very important to note that while the fluids mentioned in this article are used as power steering fluids, they are very different than conventional power steering fluids or ATFs available in most auto stores. Conventional power steering fluid or ATF is almost twice as viscous as these mineral oils and may be incompatible with the seals used in the system.
Shimano Mineral Oil
SM-DB-OIL Shimano Mineral Oil for Disk Brake($18.95 for 50ml., #Y-839 98010)(Red Color) is extremely expensive and rarely available when needed.
This is the best performing hydraulic mineral oil that I have used yet. It is also 25 times less expensive than Shimano Mineral Oil.
Pentosin central hydraulic fluid is one option. Either of LHM Plus or CHF 7.1(No. 54106) will work fine. LHM Plus & CHF 7.1 are rated at 18 cSt@40C/6.0 cSt@100C/VI 340. This is slightly thinner than the Castrol, so it may work even better in Shimano brakes. Pentosin is supplied in the hydraulic systems of most German and European cars, most notably as BMW and Audi power steering fluid. Consult a local BMW dealer about availability. Do not use CHF 11S with as it is a synthetic oil. CHF 7.1 should run about $15 per liter. I think that Pentosin CHF 7.1 is probably the best improvement that can be made to the Shimano brakes. (Green Color)
I love this stuff. I have recently started using it as a chain lube with great success. it doesn't cake up like other options i've tried.
Pentosin 7.1 is not compatible with the seals in Hayes FX-9 brakes (Seal swell).
Castrol Hydraulic System Mineral Oil (HSMO) ($13.99, 500ml., Jaguar # JLM-9886 or Castrol # 6582/5966 01) or Castrol Hydraulic System Mineral Oil Plus (HSMO Plus). This mineral oil is used in some british cars, like Jaguar and Rolls Royce, in the suspension, and brake assist systems. It is not readily available in the US but it can be found at a Jaguar dealer parts counter. This oil is rated at 19.8 cSt@40C/6.0 cSt@100C/VI 285.
Jaguar/Castrol HSMO Data Sheet (Green Color)
Other Automotive Mineral Oils
Magura supplies low viscosity MAGURA Royal Blood and Royal Blue Blood mineral oil. A 16oz. container costs $15.00 while a 2oz. container costs $4.00. (Blue Color) (reported to be 10wt. but it is probably more like 5-6wt.))
This is what Magura says about drugstore mineral oil:
What type of oil should I use in my Magura hydraulic brakes? Magura Blood mineral oil is the recommended oil, which is a 10 weight mineral oil. By far, our most common problem we encounter is consumers putting off-the-shelf mineral oil (from a drug store) in their Magura hydraulic brakes. Off-the-shelf mineral oil is normally 30-50 weight which will make the brake very sluggish. In addition, off-the-shelf mineral oil has additives, perfumes and different boiling points, all of which can cause brake failure. USE ONLY MAGURA BLOOD IN YOUR BRAKE SYSTEM. Use of any other fluid besides Magura Blood mineral oil will void your warranty. (HERE)
In an emergency, it may be tolerable to use a suspension fluid such as Motul Fork Oil (Light), RockShox (5w Medium, Hydracoil), Bardahl (5w Fork Oil), Bel-Ray Fork Oil (5wt), or Kayaba KYB (01). These may work fine in a jam. Make sure not to use any synthetic fluids. Replace with the proper fluid ASAP when you get home. The low viscosity index (VI) of these fluids are not suitable for the heat range that the brake system will experience. The viscosity range should be around ISO 18.
Can I use drug store mineral oil?
DO NOT USE MINERAL OIL FROM THE DRUGSTORE!!!! What you are buying at the drugstore is a laxative, not a specific industrial oil. This oil does not have the specific viscosities (up to 3X too thick) or additives for perfect functionality with a hydraulic system. A MSDS for drug store mineral oil is available HERE. If you were in a jam and just had to use this crap, use Drakeol® 10 or 10B, Ultraol 100NF.
Another Emergency Trick
Food grade mineral oil is also available at various high end cookwear retailers like William-Sonoma. It is used for treating wooden cutting boards and salad bowls.
I would not use this before trying a high VI suspension fluid first.