Air flow & filtering characteristics of air filter types is often misunderstood, the basics are below
Both the unifilter and K and N filters can be tight to fit on some machines. a little oil on the seals can ease installation. In some cases the manufacturing tolerances on the plastic of the airbox or snorkel can make it very difficult. Check the snorkel and air box plastic thickness around the filter seal. Some adjustment is possible.
Take care with the two self tapping retaining screws, do not over torque them. They are a little shorter than would be optimum and can strip out the thread area on the air box. It is possible to "countersink" the retainer further to increase thread length into the air box to overcome the problem where the inner is stripped.
Replacement of the self tapping screws and fitting thread inserts to the air box would be possible but the risk of them falling into the air box where a screw worked loose may make the idea less attractive.
It is important to maintain oiled foam filters properly or they will not provide good filtering or long life. They should only be cleaned with mineral turps, allowed to fully dry before re oiling and when oiling all excess oil removed. Too often excess oil is left in the filter. Unifilter Australia have guides to maintenance which are worth reading.
Experience here in Australia shows water can enter the snorkel and without any bleed hole before the air filter, saturate the standard filter. In some situations the filter can collapse.
The solution to the problem is to drill a 5-6mm hole in the air intake snorkel as per the instructions below.
A second problem with the intake system is the bleed tube in the air box intended to drain fluids from the air box. This has a 6-8 mm lip inside the air box and it is not at the lowest point of the air box, allowing a considerable amount of fluid to remain in the air box.
A drain tube with a lower lip is required closer to the outer edge of the air box. Cleaning out the excess oil is recommended at a minimum of each service interval
1/ Remove RHS "tank" cover
2/ Remove snorkel from machine
3/ Drill 5-6 mm hole as per imageDrill a pilot hole of 2-3mm first and check angle of hole during drilling
4/ Refit snorkel and RHS tank cover