Hello. I've just purchased a new Crates saddle. What is the best way to make the leather more supple? Any suggestions on products?

If it is brand new, it has a finish on top of the leather unless it is roughout. I don't know what they are using back there now for a finish, and some are more "porous" than others. You can try putting a small amount of conditioner or oil on it someplace inconspicuous like on top of the skirt under the seat jockey. If it sits on top for several minutes, then it won't do you much good to apply anything to the topside anywhere for a while. Eventually most of these finishes will wear off, and reconditioning is easier.
The biggest stiffness issues I see with new saddles, and especially the saddles from the big producers are the fenders and stirrup leathersd aren't turned. You get in them and it twists your ankles in, fight to get get any movement, and is exhausting to ride. The old fix is to wet the fenders and leathers at the bottom, twist the stirrups around. and run a stick or post through them so they'd dry that way. It is probably better to lightly dampen the rest of the fenders to prevent a "waterline" from showing up. You might also consider adding to weight to stretch the leathers some. You can bet if they didn't twist the leathers much, they didn't prestretch them either. That said, most handmakers set the twist and preset the leathers as they are making it the saddle. Some do it on their drawdown stand over a tree and others have a device that mimics the bars and has a ratchet or screw mechanism to put tension on the twisted fender and leather. You might find a saddlemaker locally who can twist and set them for you.
As far as what conditioners to use. There's a reason there about 100 of them available - nobody is clearly superior. Every one of these makers claims theirs is the best, every tack shop recommends the product that is the best, and everybody who ever owned a saddle will tell you what is best. I like some of the oils - pure NF in particular and Lexol, olive oil for some things. I like a few of the pastes - Williams Australian, Hole's saddle butter, Black Rock, Skidmores, Hide Rejuventor from Tandy is pretty good. For a finish on new stuff, I usually use TanKote. They may not be the best, but they work for me. If you have to put it on from the back side, a few considerations. Leather is porous to a point. The closer you get to the topside the denser it usually is. Anything you put on the back might wick in pretty fast, but take longer to move through the layers the closer it gets to the surface. If I apply an oil from the back only, it might take 4-5 days to equalize out through the layers. If I can apply it to the front, 24 hours will be pretty even. Heavier pastes won't wick in much until they are warm enough to liquify. A word of caution, oils on the flesh side might pull right in immediately and you think WHOA, I need more. It is better to give it some time to equalize out. You might be filling in those bigger spaces between the fibers, and as it pulls up to the grain side be just right. Add more immediately and pack that fibrous area, and the topside will be a soggy mess in 5 days.
As far as how much to use, be sparing. Most of the new production saddles are built for economy of business. They don't use as much oil and let it even out like a handmaker might. It might take a few applications and a few weeks to get it right without overdoing it. Once it is conditioned, you can take the fenders and leathers and roll them back and forth against themselves to soften them up and break them in. Onn new stiff unconditioned leather you can crack the grain doing this, but damp or conditioned leather should be fine. Hope this helps a little.

Bruce, Thanks for the tips! The stirrup leathers/fenders have been pre-turned. I bought a name brand saddle (with FQHBs) off the rack because the horse is still growing. He will be four in May. I have a custom saddle for my older horse. There is a huge difference between the qualilty of leather and workmanship. I'm hoping to get two years out of this saddle. With my custom saddle, the saddle maker told me I would probably not need to oil it for a year. Thanks again for your help!