A device for a cleaning unit for hydraulic oil or lubricating oil circulating in connection with a drive unit, wherein the cleaning unit comprises a cleaning tank provided with an oil inlet for untreated oil, means arranged to spread the untreated oil over a substantial portion of the cleaning tank cross-section, an inlet for introducing gas into the untreated oil, and an outlet for evacuation of gas and water vapor, and also a means having a large surface area arranged downstream of the spreader means, wherein the means having a large surface area is provided as a plurality of bodies arranged to be able to float on the treated oil.