I've been using the K&N RE-880 filter, but my silicon reached 9 after only 5,000 miles, which I felt was a little on the high side for no more miles than that. I added the foam pre-filter, and dropped it to less than 5 in 5,000 miles. With all the debate on K&N filters, I recommend those using them should have at least one oil analysis done just for peace of mind.
When I researched John Deere tractor filters, it looked like a basic design parameter is HUGE filters for relatively smaller engines, to keep the air velocities low enough that they can drop out some of the dust into a "sump" and thus get longer filter life. By the time I found a filter for a tractor engine of 5. 9 liters or larger, it was too big to fit under the hood, so I temporarily dropped the project. I'll be re-reading the above posts with great interest, because I think you guys putting tractor filters in are really on to a great idea, especially for those of us who find ourselves following someone on a dirt road!
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2001 ETH, QC, LWB, 4WD, 3. 54, SPA Pyro & Boost, K&N RE-880 w/foam pre-cleaner, ARE Z-series Cab-high shell, Rear ARB locker, and all kinds of synthetic fluids...