I have used Zaino in a bottle for a long time on many cars and it's one of the best products to use.
The only caution I'd have for the M5 is to not use too much and apply it very lightly with a soft MF towel as the leather (Merino) is not coated and will absorb "fluids" easily. Using too much might make the leather glossy.
For cleaning the leather I use Swissvax Leather cleaner which has worked great for me for many years. It's easy to apply and doesn't leave any streaking / gloss behind
http://www.swissvax.us/merchant.mvc?...e=LEATHER-CARE
I also use Swissvax Leather Matt for the dashboard and upper door portions.
http://www.swissvax.us/merchant.mvc?...e=LEATHER-CARE