4-Oz Finish Line High Performance Mineral Oil Brake Fluid

IMHO its foolish to use this Finish Line fluid in Shimano or Magura brakes. Perhaps a tiny bit less foolish in the Magura case, since their branded mineral oil is so expensive. Shimano, on the other hand, is pretty cheap: $11 for 100ml, which is about the same amount as this Finish Line four ounce bottle. Or $22 for a liter, which is half the price per ounce compared to this Finish Line on sale.

Mineral oil is an extremely broad term. The additives and chemicals that are included in proprietary fluids from Shimano and Magura are very specifically matched to the seals in their brake systems. These seals, especially those at the piston in the caliper, are sensitive to the oil compounds and critical to the brake feel and performance. A relatively small change in the swelling (or shrinking) of the piston/caliper seal can significantly change the piston/rad retraction behavior and feel at the lever, among other things.

No doubt this isn't rocket science, and if Shimano released their oil spec, a third party company could certainly manufacture a compatible fluid. But Shimano has never done that, and any claims of compatibility on the Finish Line bottle are arbitrary (Shimano has never authorized, approved or stated that any third party fluid is compatible with their brakes; neither has Magura).

Here's a quote from a brake engineer at Shimano, when asked if other fluids can be used:
Absolutely not, mineral oil is an unfortunate term to use because it is so vague and generic. The term mineral oil can describe a lot of different compounds. It's kind of like the term salt, which refers to much more than just table salt. Most of us are probably aware that the mineral oil that you buy at the drug store is not the same as the mineral oil we use in brakes, but that extends to different brands of mineral oil brake fluid as well. We should really call it "proprietary brake fluid" but I guess that doesn't have the same ring to it. I can tell you with absolute certainty that Magura fluid will destroy Shimano brakes in a very short amount of time. I've seen it happen several times. The rubber seals in the system have to be specifically designed to interact with a specific brake fluid. If you use a different fluid, the seals will interact differently. Specifically, when you put Magura fluid in Shimano brake, the lever feels spongy and the pad contact point changes because the square edge seal at the caliper is breaking free from the piston at a different time.

To be absolutely clear, this is NOT for automotive use. It's not high temperature rated, nor is it a glycol base like cars need.