You use a soap to clean the muck off/out of the leather, then you want something like lanolin to replace the oil content in the leather.
Propert's Saddle soap is the old standard soap, comes in solid translucent amber bars. Rub them on a wet cloth, rub the leather with the soapy cloth.
Then there is Absorbine "Horseman's One-Step Original Cream" which is a cleaner/re-oiler mixture. It's good for preservation, but less good at cleaning dirt out, you really don't want to get the dirt residue mixed in with the oils you're applying.
They leave the leather with a slightly tacky surface, which is good on saddles. You want some adhesion between the leather and your britches for security reasons. Slidey shiny surfaces look good but cause you problems if things get a little, um, dynamic.
The solution for having a shiny saddle and secure location is in britches with suede, clarino or rubberised cloth seating areas - referred to colloquially in riding circles as 'sticky bum britches' which for some reason isn't a term used in the maker's adverts
David