After looking and looking for ways to get my very stubborn orangey saddle to darken, I tried soo many, mink oil worked best for me. I think I got the method from COTH, if your interested you can search it.
* besides helping deepen my saddle it made it really soft. I used it when I first got it and still use it every so often.
otherwise I'm addicted to Antares Leather conditioner, nothing compares.
I use Neatsfoot Oil, but I probably only oil my saddle 2-3 times per year. The rest of the time, I just clean it and apply a nice thick layer of TC Saddle Buff, which I believe is just a glycerin based mixture.
I use mink oil for all of my leather tack. Darkens the leather very nicely and I use a paste so it's easy to apply with a small sponge, (easily found on Ebay). There are some threads about saddle darkening on here that describe the process...
I oil my saddle only when it needs it, (usually 2-3 times a year), and I'm careful not to overdo it. I make sure the saddle is thoroughly clean, (being sure to remove any dirt, grime, etc. with a soft cloth and water, not just caking on saddle "cleaner"). Then I apply the oil to the warm saddle, (I put the saddle in the sun or blow dry the area I'm working on), so the oil sinks in better. After the oil is all sunk in, I apply Passier Lederbalsam to seal it. It does leave the saddle grippy and a bit tacky, but I love that! This method always leaves the leather really supple and soft. The better leather quality, the better it works!