I have a single Setrab mounted up by the usual oil filter location (alongside the filter) and it really works, especially with the fan running. I can watch the temp drop when I turn on the fan. When I stop after a highway drive, the whole rear of the fender is warm to the touch from the heat coming off the cooler.
John C. says in our cars the single is good for a 20 degree F drop in oil temps, and the dual cooler is good for a 35 degree F drop.
http://vwparts.aircooled.net/category-s/887.htm