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. Oil is pumped out of the fryer vat and passed through a filter before being returned to the vat. It is not possible to completely remove all the oil in the filtration equipment each time; oil from a different vat is filtered but the equipment is flushed with fresh oil each time.
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.