There is way more to oil than the right viscosity. Engine oils have detergents, rust inhibitiors, viscosity index improvers, and tons of other additives that it needs to stand up in an engine environment. The same applies to ATFs, gear oils, and basically any other oil. Different base stocks and addtives for different applications. Hydraulic oil does not have to deal with combustion byproducts that engine oils do, so they don't have any additives do deal with them. Also, as grbily pointed out, it's not even close to a 50 weight engine oil. They use different scales for every kind of oil. A 90 weight gear oil, for example, is about equivalent in viscosity to a 30 weight motor oil becasue they use different scales for engine oils and gear oils.
In short, DON'T DO IT.