What is the best wood protection for the interior and exterior? And the best crack fillers?
Do not fill the cracks, called checks in the logs. What happens is the logs are always expanding and contracting from heat and cold and will pull away from the caulk making a hairline opening between the wood and caulk. The water can seep down in behind the caulk, but now that you have filled the majority of the opening, it cannot dry out and will cause log rot. It is better to leave them open and apply a little Shell Guard borate concentrate into the larger ones. One exception is if the check is running into a corner, door or window jamb and allowing wind, water or bugs to get in then you must fill them. Just be sure to inspect them for failure 2-3 times per year.
My opinion for protection of the logs is to use Life Line stain from Perma Chink. Either the Ultra 2 or the Ultra 7, both work great and are easy to maintain. Call Dave at 800-564-2987 for more help and the materials that you will need.