Yesterday I cleared out the leaves and muck from the airbox (under the snow guard - not sure it had ever been emptied), then replaced the dirty air filter with a RamAir ProRam dry (non oiled) panel filter...
Then went on a 160m round trip to Yorkshire to visit family today
Did a solid 51mpg average over a mix of road types, so no discernible difference Vs before, and I wasn't driving for economy shall we say!
No drone at all, even at motorway speeds, just a nice little note of induction noise when flooring it - possibly placebo but felt smoother and picked up slightly better from high gears/low rpm too
Either way, for less than £30 it's better than chucking a paper one away, and I can clean it as frequently as I want