Thanks, lots of great information, should be of value to many readers of this thread. Just for discussion, I decided to drain the rear differential with the car on the garage floor, I didn't even raise it on low ramps, so I had more difficulty developing much torque with my outreached arm I just looked up the torque, surprised to find it is 30 ft-lbs. I carefully watch all fluid changes, oil included, by putting a clean piece of cardboard under the drain plug area and coming back in a couple of hours and look for any drips - of course I wipe the plug area well before putting starting the drip test. This was my first rear differential change and the only other person to have their "hands" on my Forester is the dealer, and never for service. But when they get the car in they do a general check over and come up with $500 + recommendations for fluid/grease change. I always figured that was a good deal, they are looking for more profitable work and should be good at finding real problems too. Now here's one for you, given what I just said, I yesterday pulled the glove box to gain access to the cabin air filter. I figured it would be time to change as it is recommended at 30K and the car has 55K miles. Wow, open the filter housing, no filter! Now where did it go? Anyone's guess, but the only two I can come up with is 1) factory error, 2) dealer took it out and forgot (?) to put it back during one of their look for work efforts. The car has been to the dealer twice, once for tires and once for a recall. The '04 Forester has a "crush" washer on the oil drain plug, and when not changed/replaced at least for a few (I wasn't counting) drains it may start to drip very slightly. I was in Advance Auto yesterday to buy ATF to drain/fill the auto transmission (just the pan, no power flush)and asked after purchasing some new crush washers. They had some, per their computer, but had trouble finding them. They must not be a big seller. I have never replaced an oil drain plug washer in any other car and I've kept one of those to 200,000 miles. There may be an oil drain plug replacement with a better washer arrangement. I suppose the auto transmission drain plug is the same. But as this is the first time it is off, I'll just reuse it.

We see this problem all the time and more than likely your Return Duct leaking or sucking in the dust, since the supply air is blowing air OUT, your vents, and the return duct sucks in and I'd bet it's the area directly behind the filter grille. Is the wall cavity above and below the grille covered? also are the panning edges and headers sealed with caulk or tape? If they're taped, then that could be your problem, tape doesn't last like caulking.

A dirty engine air filter will indeed cause a car to run poorly, lack power, burn too much gas. The car "breathes" through the air filter and a clogged one restricts the air coming in. But a dirty engine air filter has nothing to do with the car's heater. The heater runs off the warmed coolant (yea, sounds backwards). As the car warms up, the warmed coolant supplies warmth to the interior heater. Some cars have cabin air filters and maybe that's what they were referring to. But if the fan blew out air that wasn't warmed, the no-heat problem was unlikely to have been caused by a filter.

I found how to fix a late model clothes dryer that needed a new heater element; and more recently how to fix my electric corded lawn mower (a $5 part that took 5 minutes to install), repair shop would have charged over $70. I learned how to disassemble, clean and reassemble various vacuum cleaners also. BTW in the process of learning to repair the lawnmower, I learned that it is important to to open the cover and clean out accumulated grass clippings - I was shocked to see all that were in mine - because they can actually get into the motor eventually and ruin it.