I've only been doing this for about a year but I've worked with a lot of saddles in that time. Certainly some flexibility can be returned to leather by oiling. But in my experience on leather that is "stiff as a board" you would have to add way too much oil to get flexibility. If the leather is stiff, but not flaking or degraded in other ways, then sometimes laying a wet towel over the stiff parts and covering with a plastic bag for a couple of hours may soften the leather. There are products in the Preservation solutions line that can then be added that will migrate into the leather, replacing the water, and allowing it to maintain the flexibility.
One of the several potential problems with this, is that you have then raised the humidity tremendously and it's possible that mold spores will be activated. Also, you should not use water on leather that is flaking or damaged. And no product can eliminate or reverse the effects of red rot.
There is definitely a point at which leather is beyond being able to be salvaged. You should take into account what parts of the saddle you are working on. Degradation in the rigging is a safety issue. Worn leather on the seat jockey may or may not be depending on the riding conditions and who is doing the riding. I find the feel of the leather and how it is reacting to bending, stretching, etc. a better indication than what it looks like.
Hope this helps,
Ann