Just to throw it out there – my personal favorites are castile soap – using very little water. It doesn’t strip the oils but gets off the sweat and dirt and water all of which dry out tack and harness. I have use castor oil for years to soften/protect (some of our tack have been used almost every day for 30+ years) – doesn’t rot the stitching like Neatsfoot oil (which does soften nicely) – but can darken lighter leathers, although usually not noticeably. Once everything is up to snuff – or maintained regularly, meaning cleaned every day after use, the classic and most popular is glycerin soap, again with little water. (PS with very few exceptions oil on riding boots). We use this process on our collections at the museum with success as well – and inexpensive too!