IMO the wattage is overkill. Assuming that the product works as described I like the idea of heating the oil rather than the water for moderately cold conditions. An expert should give you a better suggestion but I would think that closer to 100 watts would be better. You reach a point where the returns start diminishing rapidly.

For extreme conditions I think that this in addition to the water heater would be best. That way you have both coolant protection overnight and flowing oil when you start the engine.

How about putting this pad on a timer? Leave the coolant running overnight and turn on the oil pad an hour or so before you plan on starting the engine.