I have a Reserve, and the manual only recommends washing leather with a 5% soap solution, with nothing specific about the Signature seats.
Someone else posted here that the seats have a protective coating and are not raw leather (as my '63 Lincoln was and required a bucket of Saddle Soap periodically).
That being said, I've vacuumed mine off with a soft brush attachment, washed with a mild soap solution, wiped down with clean water/microfiber cloth and dried with a clean cloth. The manual recommends doing this twice a year (Pg. 6-65). I also keep a cloth in the console to wipe off any rain water that might get on them (as recommended in the manual).
I applied Mother's VLR (Vinyl/Leather/Rubber) as recommended by others, wiping off any excess with a clean cloth. It made a difference in appearance when wet and an incremental difference when it dried...but it satisfies my need to "apply something." They looked clean when I was finished. In fact, I used the VLR on most of the interior except the plastic parts and the carpet (meaning the dash, the console, and the door panels/armrests.)
I think the most important part is to keep them as clean of dirt as possible so it does not get ground in.