Disclaimer: do not use If You Have A Nut Allergy
This is the best way I know to care for leather and soften it up. It's good for the leather,easy to do, smells good and is non toxic. You can cook with the leftover oil.
The answer is peanut oil.
I've done cowboy boots, oxfords, clogs, bags and jackets. They have all turned out well, nicely conditioned, soft and supple with rich color. The woman who taught me this is a horsewoman of long standing and uses it on her saddles and other tack.
Step 1: Color Changes Vs Soft
If you have leather that needs to be softer, tack, shoes, bags, whatever this will work. It will darker the leather. Dark colors will get richer, and tans will get that dark like '40's work boots.
Step 2: Set Up
Buy some peanut oil, at your grocery or organic store and pour some in a small bowl. Work over newspaper, with a clean small size brush, like for doing trim, or basting a bird, use a q-tip,clean make up brush, or similar to test.
Step 3: Application Methods
Dip your brush in the oil and paint it on in a thick even coat. Make sure you get down by the sole and do the whole tongue. It should absorb quickly, but it will be quicker on less highly polished leather. Do several coats. As soon as the oil is absorbed, do another coat. You should be able to feel a difference after the first coat or two. The more coats the softer and more supple it will get.
For light leathers the color change will happen immediately, and evens out as you put on more coats.
You can use a rag or cotton balls or similar to apply the oil if you don't have a paint brush.
Clean up by washing the bowl and brush with dish soap, throw out the newspaper and cotton swab.