I think the kind that darkens is 100% Neetsfoot oil. Lexol doesn't darken. It's one of the things they promote about it. If you've got a light colored saddle and don't want it to darken, use Lexol, otherwise use Neetsfoot oil.
OK - good. I remembered there were different kinds of oils, some that darken, some that don't, I just didn't want to go so far as to name one... ;~D
My old Stubben english saddle was originally a light reddish color, and when I bought it used, it was also unevenly worn and faded. I oiled it down dark, so the whole thing was a dark chocolate color, and it made it look like a newer saddle - at least even!