Aside from wiping my pipes down with a Dunhill pipe wipe, I haven't used anything specific for the stems. However, since the start of 2025 I've been using mineral oil on the stems. I apply it and wipe off the excess as part of my post-smoke cleaning ritual. I can't tell yet if it is preventing oxidation, but it makes the stem look good and it makes me feel like I'm doing something good for my pipes so I'll likely continue to do it.