While we are on the topic of oil coolers, check out this N54 thread. apparently there is a thermostat that allows oil to flow through the oil cooler only when the temperature is >240F (i.e. BMW has made the decision to operate the engine at 240F oil temperature)
http://www.n54tech.com/forums/showthread.php?t=18470
The vendor also posted a link to the thermostat bypass, and that result in ~15F temperature reduction
Here's another thread referencing the Setrab oil cooler that was mentioned earlier
http://www.n54tech.com/forums/showthread.php?t=10753
Seems like the 'net gain' from the aftermarket oil cooler is ~25F temp reduction (because bypassing the thermostat on the stock cooler gets ~15F reduction)