All of the old VWs used the exhaust pipes to heat the cabin. An oil cooler would be, at best, a "supplemental" cooler--or one that took the place of rusted-out heater boxes on the exhaust. Takes quite a while to get warm enough to do any good, though.
BTW, I believe that there is still a hit to your FE from using the heater even when the coolant is up to temp. It will vary upon the amount of heat that your car needs to shed to stay at operating temp and the amount of heat you are putting into the coolant, and it is likely too small to measure reliably anyway, but I do think it is there.
Could be wrong, though...
-soD
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