I'm going to disagree with Rhaire. While that Catepillar study may be valid, I would imagine that those machines were operating in much more severe conditions with regard to dust etc. than one would ever see unless they did some serious off-road rallying. I have had great experiences with K&N, I would just recommend that you do clean them regularly (once or twice a year is usually sufficient) to keep them operating at their peak. I don't much about Amsoil, but I would think that since their other products are top of the line, they wouldn't make a less than high quality filter. But if it is constructed of foam, I wouldn't use it. Friends have not had good experiences with foam.
-Dave
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1994 850 Bone Stock, 1988 745t a work in progress . . . http://communities.msn.com/Daves745tVolvoSite