[rotorwrench]

I've used several different commercial hydraulic fluids over the years and have noted that most of them are lower viscosity types. ISO 150 or SAE 40 is about the most viscous type available. This makes it more difficult to source since there is a lower call out for the type.

In aviation, MIL-H-5606 was and still is used for most all light weight aircraft and it is a mineral base that is red in color. The heavy aircraft went to Skydrol years ago since it is temperature stable and synthetic with fire resistance being a necessity. The stuff will strip paint though so leaks have to be repaired quickly.

Commercial fluids generally are not going to be used in areas where flammability is a problem. Most are still mineral based. Tractors and heavy equipment use large quantities of the stuff so synthetics are generally much more of an expense than necessary for equipment that uses such large quantities. Synthetics are being used more and more but still not as common as mineral based fluids.