Air Filter
 

A clean air filter means more air gets to the engine more easily. It seems the best air filter is the K&N ones, it claims it can increase horsepower by 1-2. Even if it didn't increase hp, would this also help mpg a bit because it's so efficient?

in theory yes, because a restrictive airfilter restricts air just as a throttle body, and a less restrictive filter COULD reduce throttling losses compared to a more restrictive one, BUT the only way to know for sure would be to run some A-B-A tests, though i'm sure it would be insignificant

A high quality disposable paper air filter will out flow and out filter a K&N.
BMW E30 air filter tests

http://www.bobistheoilguy.com/airfilter/airtest1.htm

That's what I was thinking, difference would be small, but every little bit helps, Thanks!!

That and a more efficient air filter will help if it is the biggest restriction in the intake tract. However it is not. The throttle plate is the biggest restriction at partial throttle settings, you know, the ones you use most of the time when driving.

look up the conversion of inches of water to PSI and you'll see the difference is miniscule at best. I'm not doubting the results, but put them in terms that we readily use and you'll see the difference is little to none. besides, the OEM filter on a BMW is no doubt of higher quality than most cars being hypermiled.

(.26) inch water (4°C) = 0.009392835 pound-force/square inch.

that's the spread from worst to best.

MANN/Mahle makes air filters for most cars using the same material. All you really have to do is avoid Fram and you pretty much beat a K&N.
Exactly. So get the paper filter, save some money and get better filtration. There is no reason to waste your money on a K&N.

There are also comments on the infiniti Forums that the oil from the K&N can affect the MAF sensor.
and that is a bad thing.

It can, wire type sensor is more prone to damage, occurs mostly when filter is soaked with oil.

But it is awful lot of work to wash and oil filter every few thousand miles, if not done, then it becomes almost same as not having a filter in first place.

My car's original filter is made from foam, similar to one found from mattresses or some car seats, there is paper version for smaller engine models that does fit and I have been using that, because it is a lot less of problem.

Mine is diesel so there is no losses from throttle butterfly, however only thing I have found is that car is lot noisier when using that foam filter, boost is not coming in any earlier. I did tested some K&N filter as had one lying around, boost came on bit sooner, horrible noise and probably worse filtering, also probably increases diesel consumption as it was impossible to drive normal speed without going to boost, but with paper filter that is very easy.

As you know in old diesel rpm plays big part from boost, I can have boost at engine braking if I use over 2500rpm and pump increases amount of injected diesel according to boost.