Any cooler heat sink style (such as the BilletCool™) or radiator style work best when they are "out in the open" as much as possible, & have moving air across or through them.  

Variables affecting performance include obstructions to the the air path.  Some models have voltage regulators, accessories, exhaust pipes, etc. that somewhat block the air path and decrease efficiency of the cooler.  Under these "worst case scenarios", you should still see a 15 degree drop on average.  

If the cooler is more "out in the open" oil temperature may be lowered as much as 25 degrees.