Mutli-viscosity oils such as 10W40 have viscosity modifiers so the viscosity remains more constant over temperature change.
A 10W40 oil acts like a SAE10 oil at 25degC, and a SAE40 oil at 65degC (at least I think that's what the temps are, but they're maybe 5deg of!).

Engine oils also have a lot more additives and detergents (with the exception of the more specialist SAE oils aimed at small engines/classic engines), which there is a possibility may cause issues with some seal types, and the detergents can dislodge drit/grime that would otherwise be sitting causing no problems.

Hydraulic oil on the other hand, is pretty much one of the purist oils you can get, with minimal additives, and certainly no viscosity modifiers.