From what I can make out in the second set of pictures, the wound looks old and has been trying to heal itself. However, the wood appears to show signs of rot. Aside from hammering on it with a rubber mallet, take a pocket knife and check the exposed wood. If it's rotten, the trees days are numbered, (even if the bark heals over the wound).
Since the wound is located so close to the ground, any kind of dressing will only compound the problem of trapped moisture. The wood will continue to rot and eventually the tree will come down.
If it were me, I would remove the tree for a number of reason, especially when you consider the potential dangers of the tree falling on your house or falling across the road and maybe hitting someone or someone's automobile.
You've got time, it's not something that is going to happen tomorrow, but it's not something you want to put off too far into the future as the tree will only get bigger and therefore harder to take down.
