Motor oil protects better that hydraulic fluid.
Yes, motor oil protects ENGINES better than hydraulic fluid protect engines.
But hydraulic fluid ... good quality ... protects pumps, valves, and hydraulic motors better than engine oil. One particular weakness of engine oil is that they have emulsifiers to mix condensed water and keep it in solution. That's good in an engine, but bad in a hydraulic system. Soot is better kept in solution. Filters are USUALLY better for hydraulics.
We want our water to settle and get drained out from the bottom of the reservoir. We want EP additives that are not ideal in an engine.
Have I used engine oil, or automatic transmission oil in hydraulics? Yes, but it needs to be changes more frequently (time, day/night temperature swings) than hydraulic oil. For a VERY FEW applications, I actually prefer Dexron III to hydraulic oils that are commonly available (where a VI of over 200 is good).
But in general, not a good idea.