I just modified the plastic cover between the grill and core support in front of the factory airbox scoop to pull more air from the grill. It will also allow for a little ram effect at speed. I will try to get pics sometime. When I was datalogging for my OV tune, my inlet temps were often below ambient at speed.
I use a K&N, but mostly because I like keeping a clean filter in my cars and it will last much longer. I keep an eye for oil on the MAF sensor element as well.
As far as dirt in my engine, I chance my synthetic every 5000 miles, or sooner if it looks too dirty or diluted, and of course check it often. Clean oil makes for a happy engine and it is cheap to change IMHO, so why keep it fresh.
Don't lecture me on Blackstone Labs oil testing, service intervals, etc. I have been working on engines all my life and I know clean oil is an engine's lifeblood. If it ever starts to get dirty quicker, I will switch to an earlier interval. We happily fill our tanks with gas for about the same price as an oil change and a good filter, so to me it is cheap peace of mind.
2018 Machine Gray Mazda 6 Grand Touring 2.5 Turbo 35% tint, OEM splash guards, LED bulbs inside and license plate, Orange Virus Tune (coming soon), Corksport intake (coming soon), modification to area behind the grill to bring more air to the stock airbox scoop with K&N filter, Maxpider 3D floor mats, G2 silver caliper coating.