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.
All good info there.
ISO VG68 and even VG32 oils are quite common in heavy industrial hydraulic systems. As you say, hydraulic oils do not have water-dispersant additives, partly for the reason you give - the pickup is not right at the bottom of the oil tank, by design.
The other major reason is, so that the oil can be run through a mobile centrifugal separator unit from time to time, to remove water and particulate debris. Some plants have their own units; others use contractors'.
Then the oil is returned to service - additive degradation/consumption is usually very low in industrial hydraulic systems, unlike in engines.
Also, the oil is usually tested to determine its fitness for further service, at that time, including additive levels. $50-100 spent on testing is a lot better value than having to replace a few hundred, or thousand, litres of oil, and wearing the disposal charges for the old oil.
It's also good practice to fit 'desiccant breathers' to the oil tanks, to minimise atmospheric water entry to the system. This is particularly critical in high-humidity environments, e.g. paper mills.
