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In probably half a million miles on different Subarus with K&N filters, I have never seen the oil from the filters causing any MAF issues, but if that's a concern of yours, I applaud you seeking a different solution.
If you take down your mileage and fill amount, the fuel log on this site will calculate your exact mileage for you. I've taken to keeping a basic calculator in my car though, because the one on my phone is a pain to use and I'm usually really eager to see my numbers after a good tank.
The number on the door jamb is a recommended pressure for best ride quality according to the manufacturer. I always thought that number on Subarus seemed really low. You can go up to the pressure on the side wall without damaging the tire. Just ratchet it up to the highest pressure within that limit that you feel safe/comfortable with. Maybe find an empty parking lot and try a few panic stops and swerves to make sure it's safe. I always had trouble finding traction for braking and steering in those cars, which is why I never wanted to max out my tire pressure. The antiquated single-channel ABS in those things is not very good.
Strangely, I always seemed to get my best mileage in those cars with a lot of junk in the back. The hypothesis that I came up with was that it tucked the suspension up into the car more for better aero, but since I didn't really test it thoroughly, it remains a mere hypothesis. It could have been that I just drove more highway when I was carrying heavy stuff. I did notice when I removed my back seat and spare tire that the rear suspension rose a few inches. I measured 10" ground clearance at the lowest non-suspension point near the rear wheels (normal is 7").
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