Do a test first with a terry towel and cotton buds to see what will move it.
Start with the weakest product and work your way up the scale.
Depends on what you carry but I would go with a 'friendly' oil remover, then move to solvex, then something like POG then white spirit, but like I said don't just dive in there throwing white spirit about the place when something less hostile might do the job.
My biggest concern here is that it could be the rubber feet leaving marks. No they acn be a real problem and even paint stripper has failed for me on one occassion, but please only use that as a last resort with full customer awareness and permission.