Being in a place that gets lots of fresh airflow (albeit behind the front wheel) I would bet that the reduction in flow caused by the screen would be unimportant.
Yes. The engine (like almost all modern air cooled motorcycle engines) is "air/oil" cooled. The primary reason for that new "characteristic" in engine nomenclature came about in the 80s when manufacturers began to install jets that squirted oil at the underside of the pistons and also (often but not always) to increase oil flow over the combustion chambers. Both places, as parts of the combustion chambers, get VERY hot and don't (usually) get any cooling from exterior airflow. The advent of using oil to help cool critical parts isn't limited to air cooled engines either. Most modern liquid cooled motorcycle engines get similar measures because it's difficult to put an effective "water jacket" over the combustion chamber and impossible to have one that cools the pistons. Using the oil to cool VERY HOT parts of the engine results in hotter oil than was common before they made specific "air/oil cooled" systems. So oil coolers have become common, also they still may not be required if the engine has sufficient cooling fins.
Still, AIR directly flowing over the engine fins is the R18s primary cooling medium. Of course, it also extracts heat from the oil cooler. But if oil were truly the critical cooling method, there would probably be a fan behind the oil cooler.
FYI: When Indian first released the 111 cubic inch V-Twin, air cooled, Thunder Stroke series of motorcycles, they put an oil cooler on all of them. Then they changed to only putting the oil cooler on the Roadmaster, which is the touring model that also has lower fairings that reduces some air flow over the engine (depending on speeds & wind direction). On my own Roadmaster, in the winter, I install a custom made ABS cover over the oil cooler, so the oil doesn't get chilled down when it doesn't need it.
