cSt means centiStokes (hundredths of a Stoke, a unit of viscosity). Here is a conversion chart:
http://www.bobistheoilguy.com/viscosity-charts/

As you can see, at 40 degrees C, 100 centiStokes = 100 ISO = SAE 30, which is the normal viscosity for hydraulic oil: that is, the same viscosity as engine oil. 68 ISO is SAE 20, which is used in some hydraulic systems, but I think is less common than 100 ISO. If you want to use engine oil you have to be concerned to have the right additives, and not have the wrong additives, for hydraulic oil. The main issue is that engine oils are designed to disperse and retain water, while industrial hydraulic systems seem to prefer any water to separate out - perhaps because they usually have tanks to facilitate de-aeration of the fluid, and water on the bottom of the tank may be harmless if the oil pickup is positioned above the very bottom of the tank. You can read the something about the issues here:
http://www.machinerylubrication.com/Read/29715/hydraulic-engine-oils

In other words, most people in your situation use engine oil, and many equipment manufacturers are happy with that.