I've puzzled over the same thing for quite a while. The standard you need to meet is MS-1209, regardless of the brand. I have only found one oil other than Hytran Ultra that lists that performance standard on the bucket, and it cost just as much or more (At Tractor Supply) than the Hytran does at the dealer. So, I just buy the real deal.
There are 3 different Hytran fluids -- Hytran, Hytran Plus (?), and Hytran Ultra, with Ultra being the newest. Each of them has a different performance standard. The dealer says they are downward compatible but not upward. In other words, you can use ultra instead of plus but not the other way around.
I think it might get to be an issue if you really push the hydraulics hard. I have an HST tractor, so go with the good stuff.
The flip side is the fact that NH has essentially the same machine as mine and they label their oil differently and use a different numerical standard for it. I think it prices out about the same and is probably the same stuff in a different bottle, but the dealers can't/won't verify that. It doesn't really matter since the price is the same, but it's just one of those things that annoys a person.