I hate to be the bearer of bad news, but unless you remove the leather from the seats and condition the "raw" back side of the leather, you really are not doing much for your seats by conditioning them. It makes us as car owners feel better that we're doing something, but unfortunately, the effect is negligible.
Leather used for upholstery is dyed with pigment dyes which hides the flaws and the grain in the leather, as opposed to aniline dyes which allow the grain to be seen and the leather to age, darken and develop a patina. Aniline dyes also allow any natural flaws to be visible, which most people don't want to see. Consumers want leather exclusiveness but with "vinyl perfection". So, they use pigment dyes, which is very much like painting your leather with acrylic paint. It coats it, rather than just colouring it.
After dyeing with pigment dye, upholstery leather is then further sealed with lacquer, or similar flexible sealer, so as not to allow spilled drinks, etc. to stain the seats, and to keep the seats from looking worn. It also makes for easy clean up...just like vinyl.
Between the pigment dye and the sealer, virtually nothing will soak into the leather...including conditioners, until this sealer coat starts to wear through or crack.... or is removed.
What benefit you are seeing is probably from the massaging action of working the conditioners onto the leather, which, by working the leather with your hands, you are breaking in the leather and keeping it flexible. This is why the driver's seat tends to stay more pliable than the others. Also, in winter, it is the nature of leather to be stiffer than in summer.
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