Good question. To ensure that our oil is free from crumbs and food debris, and to retain its quality, our cooking vats are periodically filtered using specially designed filtration equipment. The Apple pie vat is filtered first, followed by chicken, and then Filet-o-Fish®. These three vats share the same inlet for oil, which goes through a mesh filter as it passes through the filter box. These vats use the same filter box and, as a result, there may be a very small degree of oil to oil contamination. Only the Fries vat uses a dedicated filter box.
During the filtering process, it's not possible to completely remove all traces of oil in the equipment. It's therefore possible that tiny traces of the vegetable oil used to cook the chicken products may come into contact with the vegetable oil used to cook the fish products. Beef and pork products, however, are not cooked in any vats.