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The oil used to coat the filter is tacky to aid trapping dust/dirt particles. When the filter media is too heavily saturated with that coating, some of the oil is "blown" through and into the MAF and intake. Since it's sticky, some of it can collect on the internals of the MAF. Depending on how much and how often the oil is overused determines how much will be passed on to your Maf and intake.
If you are concerned, you can always remove the bellows from the front of the MAF and inspect it. If it's dirty you can used an appropriate MAF spray cleaner.
I've had my K&N on my '02 Z28 virtually since it was new. The filter is cleaned often (after each track event). I have not had any problem with the oil and only cleaned the MAF once as a part of periodic maintenance.
K&N is only one of several manufacturers that make these filters. It's hard to determine how much difference a given filter makes. Mostly, I'd say it's a matter of personal preference. Don't expect miracles from any of them.
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