I bought a set of seats from an '88 Porsche back in about 2005. They were in a car that had been parked in wrecking yards with the door windows about half way down for a year and a half in PA and MD. The mold and mildew were about 3/8" thick over ALL the leather.
I followed an example I found on a Ferrari site about that time. Briefly it was:
WASH the leather thoroughly with a good detergent and water - 3 to 5 times. Be sure the leather is thoroughly dry.
Take a white towel/rag/cloth and cut it into about 8" squares. Soak a square in lacquer thinner, then be agressive in the way you wipe the leather. Keep wiping and replacing the 8" squares until NO MORE of the 'dead' paint that is used to dye the leather comes off on the cloth. When the cloth color is unchanged after about a minute of wiping in an area, move to another panel or area. Continue until ALL the leather has been treated. (use protection for your hands and lungs, and do it in a well ventilated area).
Let the leather dry for a day or so, then start using leather softener on it (I use Leatherique products, or something called 'Soffener' from ColorPlus). When the leather softener does not soak in for about 24 hours after a treatment (it took me about 6 treatments to get to that stage), the leather should be good and soft. Let it set for about a week.
Get more 8" square pieces soak them with lacquer thinner and lightly go over all the leather once or twice to remove any surface 'softener' that is still on the surface, and to open the surface up to receive a new color treatment.
I then re-treated the leather with a 'dye' (actually paint) from ColoPlus (
www.colorplus.com) in the color I wanted for the interior. I changed from brown seats to very light grey. I apply the 'dye' as I do stain on wood that I'm re-finishing - wipe it on lightly with a rag, let it dry a day, then another light coat until the color is uniform. One or maybe two more wiping treatments should make it look nearly like new. Let it set at least week, and it should be dried and ready to use. A full month is even better before using. If done carefully the seats will look nearly new.
I used seats 'restored' by this process for 4-5 years before the car was sold, and they still looked great.
Just my experience!! I'm using the same procedure on e9 seats that were in pretty poor condition that I recently purchased.