Why do my Sperry boat shoes squeak so much even after conditioning the leather?

The thing is, conditioning alone won't always fix it because the squeak might be coming from where the insole meets the midsole or from the heel counter flexing, not the leather surface itself. One trap people fall into is over-conditioning - too much oil can actually make the leather slippery against internal friction points and sometimes make the squeak worse before it gets better. You might need to target the problem spots directly: try loosening the insole slightly or adding a thin cushioning layer underneath it, or even work some conditioner into the seams and heel areas where movement happens. Sometimes it's just the shoe breaking in and the squeak dies down on its own after a few wears.

Have you checked if water's gotten trapped inside the shoe rather than just between layers? The earlier answers are on the right track about internal friction, but sometimes the real culprit is moisture that's accumulated in the insole or midsole area - Sperrys are meant for wet environments, so water seeps in easily, and when it dries unevenly it creates air pockets that squeak with every step. Try stuffing them with newspaper for a few days in a warm spot, then see if the noise changes; if it does, you're dealing with a moisture issue that conditioning won't touch.