A method and a system for purification of contaminated oil, said method comprising at least two phases: —a filter preparation phase which comprises the steps of: ∘ providing a depth filter by building at least a part of the depth filter from a filter material; and ∘ impregnating at least a part of said depth filter with a separation aid, and —an oil filtering phase comprising the step of filtering contaminated oil through said depth filter which has been prepared during the filter preparation phase.