A few coats of oil on a very old, dry saddle is just fine. I would stop there though. Now that it's got some life to it, but a nice big tin of Passier Lederbalsam. It's not cheap, but a little goes a long way. It has lanolin and beeswax in it. It will seal in the oil you've put in and protect the leather from sweat, dirt, and water. Adam's Horse Supply has the best price on Passier Lederbalsam.
I clean the saddle, let it dry, then put on a coat of lederbalsam. I let that soak in for a couple of hours. I touch up any spots that feel a bit dry after the first coat. I let that sit overnight. The next day, I use a very soft cloth and "buff" the saddle. You'll get a nice "glow" if the finish is still good on your leather.
You can use Lederbalsam on the billets once or twice a year, though I'd wait at least 2 days before riding, to be sure they're dry. I use Belvoir spray Conditioner (step 2) on my billets and stirrup leathers normally. I think Adams carries it too. I use Lexol's cleaner.
This is what I have done with my saddles for years, and I own 3 older Stubbens, a 1920's McClellan saddle, two 1950s western saddles, a 1960s western saddle, etc. All of my old saddles have done really well with the lederbalsam. They really "shine" once you get the leather really clean and conditioned well.