I use Dr Beasely's Leather cleaner. For my older cars, I went to town with a Colour Lock brush like this:
https://www.carsupplieswarehouse.com...39593408921780
...for the leather. Sometimes I use a detailing brush to foam up the solution and then wipe clean with a MF cloth.
This Dr Beasley's works fine but it has a sharp scent to it so once I'm done with it, I plan to either get the Sonax foaming cleaner or get the P&S Interior detailer. I heard the P&S can be used on leather so I think I'll do that but with the detailing brushes instead of the Colourlock brush.
The Colourlock brush, though, really restored the steering wheel on my 2008 535xi's steering wheel. I'd just been using a detailer on it with a MF cloth but noticed the wheel had become sticky over the years and sometimes I was pulling black rubbery stuff off of it, thinking the leather was deteriorating. Then I used the Colourlock on it and tons of those black flakes were falling off. I figured I'd royally F'd up my wheel's leather and just said F it, and kept going.
Turns out, that was 13+ years of skin debris and oils from my hands on the wheel coming off. I went through 3 or 4 MF cloths wiping that stuff off and wheel's leather was actually just fine underneath all of that.
I'm not a messy person but that grossed me out. I did the same on my seats and while some blackish dirt and grime came off, it was not nearly as gross as my wheel.
Now I plan to clean my steering wheels on all my cars every several months so it doesn't look/feel like my E60's wheel did. I thought just wiping it down with a detailer every few weeks was cleaning it but turns out it's just wetting the debris so that it can cake up when it finally dries. You need to use a brush.