I have three of B-17s (two were gifts), and I haven't heard a squeak in what's probably a combined 15,000 miles or so.
Have you rubbed it down with Proofhide, Brooks' leather gunk? Usually you apply that minimally, as little as you can get to spread over the whole surface, once or twice a year. In this case, though, using it on the underside of the saddle, and massaging it in wherever metal touches leather, might help. Brooks is pretty definite about using non-Brooks treatments on those things, which I imagine is mostly hype, but Proofhide works and it doesn't cost any more than any other good leather treatment.
If you do decide to oil the squeaky places, I'd get something made for leather, like mink or neatsfoot oil, not just whatever's in the can. Obenauf's makes a good leather preservative (same consistency and smell as Proofhide, in fact) and also a good oil. I've used both for years on all kinds of stuff.