Regarding the oils, one official cross reference I found was Texaco Rando HD (only reason I remember that, is that's what we used to use at work before switching suppliers).
What Rando HD is, is a pretty standard hydraulic oil with some basic additives, for use in most normal hydraulic systems.

The slideway lube you linked to will most likely have tack additives, so it sticks better, and I personally wouldn't be putting it in a gearbox.

Try some of your local agricultural merchants/dealers, as they normally stock hydraulic oil, although 68 is not a very common oil (32, 46 and 100 are all far more common)