I hate to say it, but have you tried diaper cream? The high zinc oxide ones are basically a graphite lubricant for your butt. Silicone anal lube? The Desatin extra something or another is what is usually recommended. It's basically 40% zinc oxide in a froth of petroleum jelly and mineral oil. Absolutely frictionless. We use the California Baby hippie version of that because we're, well, hippies.