Grant Petersen at Rivendell recommends Obenauf's Leather Preservative, which you can get at many boot places. I've used both and can't tell a difference (one of my B-17s is ~20 years old). But as Scott said, it just keeps the leather from drying. You don't actually want to soften it anyway--it shortens the life.
Some other tips that have worked for me:
I've found the break-in horror stories to be exaggerated. I have three B-17s, and while they do get better in a few hundred miles, all were comfortable right out of the box. Just ride.
Setup is critical, at least for me. A couple of millimeters up or down on the nose makes a big difference. Use a ruler and measure from the top tube.
Whatever you smear on, use it sparingly. I think Grant says a gob the size of a pea once a year. I live in the desert and do it twice a year. It took me six years to use up a can of Proofide, even with four saddles (I also have a Pro).
Protect it from water, and if you get it wet, don't ride it until it dries. I keep a plastic grocery sack in my seat bag in case of rain. I should probably protect it from sweat, too, but I can't bring myself to do it.
Bianchi San Remo - set up as a utility bike, Peter Mooney Road bike, Peter Mooney commute bike,Dahon Folder,Schwinn Paramount Tandem
Agree with the others, I have three B-17s, with 28,000 miles between them. All have been comfortable from day 1, but they get better with time.
The Proofride is just waterproofing - I'm sure that any saddle soap would work just fine.
Tip - don't wear white pants after a fresh coat of proofride.
Your new saddle is to new to start applying stuff to it. The factory has aready done that.
It is meant to be ridden and sweated on. All part of the conditioning process.
Shove a plastic bag under the rails for heavy rain.
I have an original B72 on my raleigh 3 speed built in 1972. Still in great condition.
For me regarding treatment, less is more.