What, in your experience, defines the point where a fluid that meets or exceeds the specifications is inferior to another? Motor oil was used in equipment for a long time, however, not sure I would try that in a Hydrostatic setup.
Just a couple of things. We are assuming that both of these fluids - hydraulic and motor oils - are mineral oils and have the same base structure. And that is mostly true for everything except the exotics like brake fluid, aircraft fluids, and high vacuum fluids.
Anyway, for a long time, one of the main differences in common hydraulic oils vs motor oils was that hydraulic oils had a lower viscosity overall. Hydraulic fluids flowed better because they stayed low viscosity at low temperatures. Getting them to do that requires more refining and is part of why they cost so much.
Motor oils used viscosity improvers instead of expensive refining, and viscosity improvers could only do so much. Their temperature range was limited. The viscosity improvers were additives and they concentrated on improving the high temperature side, since motor oils had to withstand higher temperatures.... and that hurt their viscosity at low temps.
So the guy working his hydraulics on cheap motor oil would fill the hydraulics with motor oil and when he went to crank it up in severe cold the hydraulic pump would immediately flash a starve warning on the dash. Unless our guy was doing some sort of preheat like keeping it inside a shop, the cold motor oil was too thick for the suction side of the hydraulic pump - hence the dash light warning. JD commercial equipment ran into that when they tried using their motor oil in the hydraulics 20 years ago. But recently there have been some changes in the way that motor oils use viscosity improvers and especially now that synthetic motor oils are made for a reasonable price - causing motor oil to flow better at low temps.
So
viscosity range in the cold was & is a known big difference between hydraulic and motor oil fluids that has been solved lately. Not perfectly, but it is a lot better now with the new super low viscosity synthetic motor oils.
The one spec we still don't know anything about between hydraulic fluids and motor oils is still a bother.
That remaining problem spec bothering us is the compatibility of the motor oil additives with the elastomers and some of the plated parts. This is mostly with shaft seals and hoses - that are used throughout the hydraulic system. "Compatibility" in elastomers means, "does the fluid or additives cause the seal and hose material to swell and soften?".
There are ISO, DIN, SAE, and ASTM tests for what is called "elastomer compatibility". They limit the swelling and softening of seals. My beef with the hydraulic fluid industry is that we have the tests and standands availiable, but the hydraulic fluids do not require those tests in their hydraulic fluid "standards". So we cannot know for sure. We know that motor oils have pretty good wide compatibility, but we just don't know about hydraulic oils. My suspicion is that the compatibility tests are expensive to do and to test for.....nd that cuts into profits. It also provides a entry point - a "low bar" - for cheap hydraulic fluids.
And so I finally get to the one reason why I spend the extra dollars for name brand hydraulic fluid. Not because I know it is any better, but I hope it is. I am basically gambling that the name brands do test for compatibility because they have more to lose if they get it wrong.
You may say my reasons are based is pretty poor logic and I would agree. Gambling is a pretty poor reason for me to be spending the extra on higher priced name brand hydaulic oils. But it is all I have to work with. YMMV....
rScotty
