I just learned about the Messerschmitt BF-309:
i.pinimg.com/474x/c3/e6/9f/c3e69f2bebfef3c923348b6924930687--military-aircraft-wwii.jpg
This picture doesn't show it well, but the variable flow oil cooler is modulated by the scoop extending and retracting in the air flow. Compare with the Mustang that has the same added drag whether the cooler is open or closed.