I think you are wise to be careful, many abrasive cleaners will attack the surface of the medal. I think your best bet would be Goddards Silverdip, I'm pretty sure it's non abrasive, you just dip and rinse without need for nay form of rubbibg. If your medal is very dirty, it may need multiple treatments.
I'm sure there are a number of members here who will have medals, they'll soon put me right if I'm talking rubbish.
The gruesome twosome, Brumas and Bingalau, I would think might have some ideas. ;o)