Bore oil helps protect the bore of your instrument from moisture. It does not go rancid and provides lengthy protection.
You should oil the bore of your instrument couple of months with a small amount on a clean cloth wrapped around a cleaning rod. Be sure to avoid getting oil on any pads or corks, or applying too much. Allow to sit for up to an hour before removing excess.
Do not use cooking oils or other oils as they may go rancid, damage the wood, or not offer sufficient protection.