Sure, they may seem straightforward enough, especially when you look at them in a clear glass; oil is oil, after all. In short, these fluids are highly specialized for different environments and not interchangeable with something like motor oil. They both have numbers in the name, for one, and sometimes hydraulic fluid even has a "W" in it. But that W stands for "Weight," as opposed to automotive engine oil where the W stands for "Winter." And that's just scratching the surface between these two, because each fluid has totally different additives that enhance their respective properties.