All air filters are a compromise. Filtration is always indirectly proportional to air flow.
Oiled air filters attempt to provide improved air flow with less of a compromise on filtration by oiling the media. But, as others have already pointed out, YOMV
Too much oil can cause intake gunk and deposits. Too little means the wide open fabric of the filter will not filter as well as is optimum. The obvious down side to the latter is possible premature wear on internal engine components.
The real question here is: What do you hope to achieve by switching from a stock to an aftermarket filter?
The stock filter works pretty well. When clean it is a reasonable compromise point between good filtration and good air flow. But it does filter, and therefore it does restrict airflow somewhat. What this means is that when the filter becomes a significant limiting factor in the intake airflow, the engine power will be somewhat reduced. When does that happen? Only when the throttle plates are NOT the limiting factor to airflow. IOW - only at WFO throttle, and especially at the highest rpms, when airflow is increased.
Are you looking at getting the highest power at very high rpm and wide open throttle? Will you be drag racing your FJR? If not, the air filter is not the major air restriction, and you will realize no appreciable increase in power. If you are looking to drag race the engine and are only interested in maximum air flow at high rpm, then even a K&N filter is (still) a compromise. Just pull the air filter out altogether for the maximum possible airflow. Yes, you will end up having to rebuild or replace your engine more frequently, but who cares? That wasn't your goal.
OTOH, if your goal is to have reasonable performance and good engine longevity, then the best possible thing to use is the stock air filter. There are design engineers at Yamaha with way more smarts than the dipshits at K&N, who have calculated the optimum point on that compromise curve to both provide adequate filtration and still give excellent airflow (and therefore engine power) at the vast majority of throttle openings and rpms likely to be encountered.
The choice, as always, is all yours.
