Well, for what it's worth my Grizzly chinese lathe manual specifies ISO 68 gear / hydraulic oil. I have to believe that for a home machine user, who's machine tools are not being run constantly (even if you are using them a LOT), you should be fine using either ISO32 or ISO 68. The key here (he said pretending to be knowledgeable) is using a non detergent oil, as those detergents can be harmful in this kind of system. As it happens, I have been using sae 30 oil for some time, because that's all I had available, and some oil is better than no oil. My lathe is a simple taper roller spindle bearing set up, with belt drive and back gear.
However, since you started this thread, and reminded me to get off my backside and source the right fluid, I have done so.
As for source, you can get ISO 68 hydraulic oil at amazon.com - four gallons at 12 bucks a gallon. Free shipping if you're in their prime thingy (which I am), so I just bought some, and I don't see myself running out for quite some time. Think I'll drain all the various oils out of my lathe this weekend. I found ISO 32 at home depot in 1 gallon jugs at about 14 dollars a gallon too.