I change mine every oil change (or every other, depending on my mood), and these days that either four or two times a year. I live in an agricultural area, and right now everyone is taking advantage of the nice dry weather to combine soybeans and corn -- dust EVERYWHERE. I washed the cars last weekend and they are yellow now.

Air filters are cheap, change often. On the turbos, you will notice more than normal turbo lag and higher idling tubo "whine" (if you can hear it -- the Volvo sounds like a police siren a block or so behind you!). More dirt will make it thorugh a dirty filter than a clean one, reguardless of blockage, they are "depth" filters.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!