Reissuance wax works pretty good.
Can't say for sure as I do not know what the original finish is. Personally I would start with a mild solution of dishwashing liquid and very warm water with a plastic scrubbing pad. Rinse. Repeat. Rinse. Repeat. Etc.
Renaissance Wax is what Jim Supica former curator of the NRA National Firearms Museum recommended to me when My Lt. and I were helping out at the Saunders Museum with a new curator there in keeping the extensive firearms collection in good shape. It is what I have used ever since and it cleans and protects better than anything I have used in the past.
