FERIC tested four bypass filters to confirm the results of previous studies, in which bypass filters extended engine oil-change intervals and kept hydraulic oil cleaner than standard filters. Each brand of filter in the current study increased the oil-change interval, with a potential payback period ranging from less than 1 year (filtering engine oil in machines returned to a garage for oil changes) to 4 years (in a hydraulic system whose oil-change interval was extended from 1 year to 3 years).