I have an older 911 with black leather interior. I had to replace the leather on the bottom of the passenger seat. I used OEM spec leather and had it done at a reputable local car upholsterer. Looks great, like brand new.

That's part of the problem. Now the passenger side seat cover is much less shiny than the driver seat. My upholsterer tells me that the only way to make leather shine like the other seat is use and age.

Any ideas on how to accelerate the process?

 

Try giving the other leather surfaces a good cleaning. Use a soft bristle brush, spray cleaner directly on leather, scrub gently to a nice foam, then wipe with mf towel.

http://www.glisteningperfectionstore...natural/Detail

That should help.

 

As intimated keep the leather upholstery surface clean and free from abrasive rit is of tantamount importance.

Shiny Surface – OEM leather upholstery should have a matte look. If dirt and / or grit are allowed to remain on the surface it will abrade the surface and make it ‘shiny’, eventually causing the leather to split along the crease lines, dirt in these crease lines also makes the surface appearance appear worse than it is, especially on light coloured finished leather.

So keeping the finished leather surface clean is very important. If the finished leather has become excessive the pigmentation and / or the urethane can be renovated.

 

Yeah, your old leather is dirty. The shine is body oils, sweat, whatever that has collected as a film on the surface. Adding leather conditioner to these types of seats will only make it worse - the conditioner will sit on top of the existing dirt. You'll need to thoroughly clean the seats with a leather cleaner such as leather masters strong cleaner or a properly diluted, leather safe all purpose cleaner. You can use a stiff brush on leather without isuse. After a few cleaning passes the leather should turn almost tacky, which is when you an apply a good leather conditioner. My favorite is leather masters vital followed by protection creme.