I had one of those filters. If you look up
Tuned intake it'll help.  I modified the box to increase the volume of air between the
filter and carburetor. The general idea I got out of it was that the volume between filter and carburetor needs to be equal or greater than the volume of the cylinder. I had the box in my hands so that I could get the dremel in and seeing the innards let me cut out a bit of plastic wall
IDK how they built it. It ran well but I don't know what changes I made that helped me because I didn't test,reassemble, test,disassemble,make a change, test reassemble
Etcetera Etcetera, Etcetera...
It's good to keep your old stock filter, so you have one to use while the K&N dries.

Without the plug in the airbox drain you have been pulling the worst kind of crud directly into your engine.  All the trash, grit, sand, dirt, pebbles, water, etc. directly off your rear wheel. That probably didn't do much good.

The significance of checking how much oil accumulates relates to diagnosis of your problem.  If you don't accumulate oil in the airbox, then it is most likely not an engine breather issue.  The oil only has three ways to exit the engine: leakage, breather system, consumption.

Leakage will be obvious.  Puddles under the bike, goo all over the engine, etc.

Breather system should manifest in large amounts of oil accumulating in the bottom of the airbox.

Consumption is a bit harder to nail down.  It either leaks down the valve stems/guides, or gets pulled past the rings.  

I would run it with the plug installed for a good long time.  Start with the oil level normal.  When you have to add oil check your air box drain.   That way you know how much you used and can compare that to how much has accumulated in the airbox.

As far as checking oil level goes, I always use a small level across my starter to get the bike exactly level.  I also always check it when the bike is upright on my poor man's stand (an elaborate system of 2x4s).

Yeah found some cool stick on & magnetic bubble levels on amazon. An interesting addition to the bike. Glance at the level & glance at oil level sight glass , super simple , Tanx

Mine are a pack of 4 and I have some 3M 2 way tape ta stick it on. It’s more to make people think , Why ? A Tiltometer for those afraid to lean  ,  Lmmfao

Yup that’s it a Tiltometer    

I agree Versy, you don't want to tip the bike over.  I never have liked putting a block of wood under the kickstand.  That's why I always park my jalopy on this simple stand constructed from scrap lumber.  The tie downs permit accurate leveling of the bike. It works good, is safe, and was essentially free.  Makes my life a whole bunch easier.

It's a very simple stand.  You just roll the bike on and fasten it with standard motorcycle tie down straps.  I love the thing.

To make it a bit more luxurious I stacked a few sheets of old rubber material onto the bottom, but used it for many years with just bare wood.

It's not like I'm anal retentive over the oil level.  Typically, I just want to know that it's in the bullseye.  In the past, I never had to add oil.  But recently, it started to use a bit.  That's when I got serious with the level checks.  I wanted to know how much it was using.  Only way to do that is make sure it's in the exact same orientation each time you add oil.  Otherwise, it's a crap shoot.