The short answer: how to clean a leather couch depends on which of two leathers you own, and almost nobody asks that first. Protected leather has a pigmented topcoat sealing the surface, so it tolerates a damp cloth and mild soap. Aniline leather has no topcoat, so water sinks straight in and leaves a dark ring, and soap strips the oils that keep it supple. Put a drop of water on a hidden spot: if it beads, the leather is protected; if it soaks in and darkens, it is aniline and needs a leather-specific product only. That single test explains why one guide swears by warm soapy water and another says it will crack your sofa. Both are right, about different sofas.
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