The most important thing I've learned is that most leather in the interior of our cars is FINISHED LEATHER. It has a protective coating on it that conditioners can't really penetrate. So, applying an expensive conditioner can be a waste of time, energy, and money. Also, many times there are matching vinyl parts and it can be tricky to tell one from the other. You don't want to waste leather conditioner on vinyl.
According to the pros I've talked to, the best way to treat finished leather is to keep it hydrated by wiping with a damp cloth once a week. I like to use Leatherique Prestine Clean for this as it hydrates and gently cleans. It works great on both finished leather and also vinyl so you don't have to keep track of what's what. The last time I used Leatherique conditioner it just built up on the surface, never really soaked in, and remained tacky. After that I decided not to use it again.
If you know for sure you have ANALINE leather (like a King Ranch interior) then you need to apply leather conditioner.