Just to chime in, I have spent a lot of money and time wasted trying to get GP leather seats up to par. Finally had some decent luck.
I had bought this
http://www.leatherique.com/do_it_your_self_instruction/rejuvinator_pc_directions.html
I had great results by the following method:
1. take a sponge soaked in HOT water and get the crud off the seats.
2. park your car in the HOT ass sun.
3. apply the oil THICK. rub in it well with your hands on all the leather surfaces.
4. close the doors and go have a few beers or whatever you do.
5. after a few hours of good heat soaking, grab a clean scrub brush (like you would use in your bathroom) and the prestine clean and go to work. you can scrub them lightly with just the oil, then spray the cleaner on and scrub like hell. apply more prestine clean, continue to scrub.
6. using a white cloth preferably, wipe it off. be amazed how dirty your seats were.
7. you can either repeat, or just spray the prestine clean on once more and do a thorough wipe with a clean cloth.
It was amazing. I've never gotten them that clean and soft before. Really took the stiffness out of the seats. In my opinion, I think the trick was actually scrubbing them with a brush instead of just wiping them down. But I do really like this product. I don't however think the scrub brush is something you do all the time. Like the instructions say, a soft brush is best for regular use.
After 10+ years they do get naaaasty.