Never fill the splits (checks) in a log unless they are running into a door, window or corner. What will happen is the caulk will dry out and break away from the wood allowing water to seep in behind and not be able to dry out properly, causing log rot. When you must fill a check, it is very important to inspect them at least twice a year and reoair any breaks. This is the part that most people will not due and within just a few years there are several rotten logs. If you MUST fill one, place a closed cell backer foam in the check and apply 1/4” thick coat of Energy seal to fill.