You can get MIL-H-5606 at any aircraft or helicopter maintenance shop. It is a mineral based red coloured oil and is not compatible with seals meant for glycol based liquids, although that may not be true nowadays if the "rubber" components of your brake system are actually made of synthetic materials.
Every material (gas, liquid, or solid) that is known to man is compressible. Liquids are far less compressible than gasses, and solids (generally) less compressible than liquids. This would seem to suggest that liquids with lower viscosities may be more compressible than liquids with higher viscosities. If there are any physicists on here we may get a definitive on this.