I use baseball glove oil/conditioner made by Franklin. Its basically just a highly refined mink oil and is very inexpensive. I just buy it at big 5 sporting goods. Apply it in the evening and rub down the seats again in the morning with a dry towel. Years of regular use (once a month)and I've never had cracks or tears in a variety of cars/trucks.

I did use Neatsfoot oil for a while but it can get on your clothes.