Dry food starts to get “old and rancid” as soon as the original bag is opened and the food is exposed to air (even in a container). Adding moisture in the form of fat can speed this process up and the added moisture can be favorable to mold growth. I would add the oil at feeding time instead of mixing it in the kibble and keep the dry food in it’s original bag, with the top rolled down tight and the whole bag kept in the bin instead of pouring the kibble out. The storage bin also needs to be cleaned as fat from the kibble will stick to the container and continue to go rancid and contaminate the rest of the food and future food you pour into the large container. BTW, are you feeding kibble? Couldn’t be sure from your post. I just assumed.
