Go to a medical supplies outlet and ask for skin prep swabs (alcohol-based) or adhesive remover wipes containing siloxanes.
I use Welland Adhesive Removers, used for cleaning up aggressive surgical dressing adhesive after removal. It is very strong and effective. Once a corner of the leather is raised off the camera, the swab is rubbed in a forward fashion. I did this to a Ricoh GR1 years ago and it worked a treat; no guarantee it will work for old cameras with unknown adhesive.