Step 1: Use 50% diluted Simple Green and Scotch Brite Sponge (yellow side) to thoroughly clean the surface. Make sure your rinse the sponge frequently. If there are stubborn stains, use the green side of the sponge along with ample Simple Green solution to gently rub them off.
Step 2 : As others mentioned, Lexol is a very good leather conditioner. Use it generously along with Scotch Brite sponge (yellow side) and massage it into the leather.
Step 3 : After an hour or so, buff the leather with the microfiber cloth.
Try mixing 1/4 cup Murphys Oil Soap liquid with 5 cups warm (not hot) water. Use a clean microfiber towel soaked in this solution, squeeze out excess solution and clean all interior leather, vinyl and plastic surfaces (except view screens / displays). Do not get surfaces wet with solution. Do not use on fabrics.
After cleaning, follow up with your favorite leather conditioner (Lexol, etc.).
I use Zaino Leather Cleaner and then treat it with the Condition.
Meguiar's Leather Cleaner & Conditioner, 2-in-1 which I'm not a fan of is shinny but also makes it feel oily.
Regardless what you use, even Meguiar's Super Degreaser which I sometimes use in between since Zaino is expensive I LEARNED TO USE A BRUSH because wiping it off with a Microfibre does't really work.
Many of the suggestions here are way too harsh for the Nissan poly coated leather seats.
Please don't use simple green or a scotch brite or any abrasive on these seats, you will polish the coating off and cause discoloration.
These seats don't need treatment as they are already coated. Leather conditioner or similar will just sit on top of the leather coating and get on your clothes.
What to do then? Get a soft horse hair brush like this one: