Useful information......Hydraulic oil that needs to flow in cold weather = Aircraft Hydraulic Fluid. Long ago, I plowed the snow on my parking lot. When it got really cold like 15 - 20 below zero, the plow hydraulic system caused problems. The plow hydraulics used a 12 volt motor (like a starter motor) to run a small pump that lifted and lowered the plow. When it got real cold, the motor for the hydraulic system would draw so much current that my truck battery voltage would go low enough to cause the truck to stop running. It only happened once. From then on, I watched the battery voltage and when it started getting low, I just took a break and let the alternator charge it back up. During one of these breaks, I happened to look up and see a Military jet flying "way up there". It seemed to me that lots of airplanes flew "way up there". It's really cold up there. They have lots of hydraulic stuff on them. I did some investigating and found that aircraft hydraulic fluid looks like ATF. It's red, low viscosity, and stays that way when it gets cold, and nobody could give me a straight answer about using it for my truck plow.
The rest is history. For the next 20 years I never used anything else in a plow hydraulic system. It never caused any kind of problem. The viscosity at 70 degrees F and -20 degrees F looks very much the same. The only problem was buying small amounts. At first it was only available in 55 gal. drums. After a few years, you could get it in 5 gal. cans. It does cost more than ATF, but the difference was worth it to me.
