I had amazing results with Lexol Cleaner and Conditioner in our '90 Cabby, different leather obviously, but I just wanted to say that the product really does a pretty great job despite some youtube reviews I've seen of it. There's one a-hole in particular who really doesn't like Lexol and just makes videos intentionally trashing it.
I used the Chemical Guys horsehair brush (only out of quick amazon availability and time crunch because I decided last minute to take the car to a show) and while it worked well for the job, I don't care for it as a product. Many hairs came out and I would not recommend or purchase a second one.
I've found the key to really breaking down grease/dirt/grime that was deposited was a spray down of warm water onto the seat then immediately hit the area with the Lexol cleaner and start scrubbing with the horse hair brush. Really get the lather up. Wipe clean/dry with microfiber.
Hit with the conditioner following the instructions on the bottle.
I had people asking me at the show when I had the seats redone, they haven't been, ever.
This was my result: