As I previously posted, when I got our new '07 SLK 280 last November, the leather and plastic on the interior was just a little too "flat" for my tastes. In fact, to me it looked "dead". A friend of mine from my town suggested Malco conditioner. I "Googled" it and found several sources for it, in 936ml size for about $15.
I tried it, and it make the SLK's interior look great! Malco is about the consistency of Meguair's, but Malco contains mink oil and neatsfoot oil. These oils, by themselves, would be "too much oil" for leather treatment, and wouldn't smell good either! But in this product, there is a nice, faint leathery odor.
Can't say how this product would work on heavily worn leather, but as a conditioner on new leather, I recommend it.