A method and apparatus for extending the usage life of cooking oil used in a deep fat fryer in which the desired cooking oil usage life is determined. Upon the occurrence of a triggering event, the level of cooking oil in the vat of the deep fat fryer is varied between an upper first level and a lower second level by extraction of used cooking oil from the vat until the cooking oil level reaches the lower second level and subsequent input of fresh cooking oil until the level of cooking oil, used and fresh, in the vat reaches the upper first level.