Leatherique is widely regarded as one of the very best leather cleaners and conditioners on the market. It is a two step process, that actually "floats" dirt and contaminants up out of the grain, and I have found their products visually superior on light colored, older, or heavily soiled leather. You won't notice it's effectiveness as much on black, but I am still happy knowing I am preserving my leather with the very best. It's expensive (and goes a long way), but if you are keeping your vehicle only for a short time period you may be just as happy with a less expensive product.
*One side note - if the previous owner dyed the seats, the Leatherique may lift some of the dye.
If it's good enought for Rolls Royce and Connolly Hides, it good enough for me!
http://www.leatherique.com
A complete care guide (and history lesson) can be viewed on the attachemnt.