Here is Bobcat's official stance on using engine oil in the hydraulic/hydrostatic system.
"Bobcat Hydraulic/Hydrostatic Fluid is specially blended for use with Bobcat hydraulic and hydrostatic systems.
Bobcat Fluid has been developed to meet the growing demands placed on hydraulic components, while engine oil is designed to meet requirements for use as an engine lubricant.
Because of this, engine oil is no longer an acceptable alternative fluid for Bobcat hydraulic and hydrostatic systems.
Viscosity.
Engine oil viscosity is typically much higher than the acceptable limit of hydraulic fluid.
Hydraulic fluid is designed for high pressure applications and with viscosity to operate effectively throughout a wide temperature range.
Additives.
Bobcat Fluid contains more zinc and phosphorus anti-wear additives than other brands of hydraulic fluid.
Zinc and phosphorus adhere to metal surfaces of hydraulic components and act as a coating to separate moving parts.
Additives in engine oils do not have the same anti-wear properties.
Demulsibility, Under operating conditions hydraulic fluid is designed to separate water, engine oils typically absorb water.
Foaming, Most engine oil is not formulated to prevent foaming.
Foam is a contaminate that lowers performance, decreases filtration and can cause premature hydraulic component failure."
Now that being said many members here still use engine oil or universal hydraulic oils in their machines. I'm not going to get in to a debate about what is correct you have to decide for yourself what's best for you.