saddle soap

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A preparation containing mild soap and neat's-foot oil, used for cleaning and softening leather.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A soap which is used to clean, soften and condition leather, especially horse tack (hence its name).

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a mild soap for cleansing and conditioning leather