As written, an original filter (air or oil) has a high filtration capacity. I replace filters once a year no matter what state I suppose it is.
1) The oil filter, important point of the engine lubrication circuit. So, for me, it's original filter.
Never confronted with rust. I put in contact manually and if necessary, light tightening using a metal or rubber tape key. and if still damaged, a light veil of grease will protect effectively.
Also, be aware that a K&N oil filter slightly influences the temperature of the oil, downward.
2) A manufacturer, knowing better than anyone what is best for its engine, recommends viscosity, technology and oil quality, usually 10W40 semi-synt. because adapted to a wide range of operation. Depending on the climate under which you drive, the use of the bike and the engine mileage, it may be wise to adapt the oil.
Also, be aware that a technology 100% synt. will not have the same constancy in time, as a semi-synt. On circuit, it is not uncommon for the synthetic oil to be replaced at the end of each day of driving. For us, it must be able to be constant at least 15000 km, under all conditions.
3) the ease of replacement of an original air filter is undeniable compared to the maintenance claimed by the K&N filter, which will still require any other item of origin or other, in order to continue to use the vehicle the time of maintenance of the K&N. I doubt the financial gain in the long term, for the K&N.
4) Irridium spark plugs, although usable on our trails, are recommended for high efficiency engines. They do not accept city use (traffic jams), low speeds or even under-revs, not optimal operating temperatures that can be with a trail.
We must resonate as a manufacturer who has every interest in selling a motorcycle as successful as possible, and avoid the returns that can cost him, not to mention the word of an unreliable bike, low performance or consisting of elements of basic quality, which could spread in the world of motorcycle.
Here ! Hoping that the translator will not have too much transformed what I wanted to express, and of course only binds me.