I bought the car with the seat already in this condition. It looks like a sweat stain. I tried cleaning with detergent and even applying leather color dye. It cannot be hidden by paint. It feels hard when you rub your hand across the stain.
Any ideas??

 

Yeah, and it ain't sweat.

You can take the car to an upholstery shop to see if they have any suggestions. Sometimes it's just finding the right product. I use a leather repair and touch up kit I bought from a place out of the UK [LINK] with some success, but you need to reapply about once a year. That being said, I've never seen leather go hard, so an upholstery shop may be the best place to ask.

 

i agree with zak in indirect way.... the previous owner didnt take care of the seats. i would find a pro to look at it, don't risk messing it up worse

 

Well I scrubbed some more with a toothbrush and detergent and it seems somewhat less visible. The leather actually feels a lot softer in the area scrubbed.
What exactly do those leather restoring kits offer? Is it some type of oil to rehydrate the leather? I'm curious about steam cleaning with the leather. Wouldn't the steam help hydrate the leather?
Either way I'll try a few more minor attempts at some type of solutions before going to some type of upholstery shop.

However, if anyone has any experience restoring leather seats that weren't well taken care of, please chime in!

 

Take a look at the Leatherique products. Ive been using them for 20+ years on vintage Alfa's and BMW's:

http://leatherique.com/