[Efthreeoh]

Okay, so I set some things straight here.

Engine Air Filter: The BMW recommended engine air filter change interval is every 3rd oil change as dictated by the CBS (Condition Based Service) maintenance system. Somehow on the internet that has come to mean a 15,000 mile oil change, but that's actually false. But if you assume a 15,000 OCI then the engine filter gets changed about every 45,000 miles, which is about the industry norm for modern cars. The maintenance booklet states to change the engine air filter every 3rd oil change. A dirty engine air filter like the one you had is not a bad thing; it actually means it doing it's job. Don't worry that it went a little long in the change interval.

Power Steering: There has to be a leak somewhere because the power system shouldn't lose any fluid. If it is leaking, there are only a few places it can leak from. First check the steering rack bellows. The bellows are the rubber boots on each end of the rack that protect the hydraulic pistons and ball joint of the inner tirerod end. Just squeeze the boot. If you hear a sloshing/slushing sound, then there is fluid in the boot, which means the rack end piston seals on the rack are leaking. The other places the system can leak is at the hose connections, there are just two on the underside of the reservoir, and two that are on the rack, and two for the PS cooling hose. There are also two hard lines on the rack itself. All E90's use CHF-11S. I think Pentosin has since introduced a new fluid that meets the CHF-11 spec but has a different name. BMW AG does not recommend a change interval (most manufacturers don't), but most dealers suggest the service. They figured out people on the forums, mostly enthusiasts, change the PS fluid, so they started recommending the service because it's easy money. It's really not necessary if the system never has an issue, but it's not a bad idea to flush the fluid and refresh it. I do mine every 150,000 miles or so. I've flushed it twice. Flush the system, don't do the turkey-baster method, it is a waste of fluid; for 50 cents you just need two new crush washers and just open up one banjo bolt on the steering rack.

Stick with a Mahle or the MANN engine oil filter. Both companies are BMW OEM. The MANN HU-816 is the correct OE oil filter. Other aftermarket oil filters do not fit correctly IMO. Use engine oil that meets BMW Spec for LL-01. You change interval is up to you. Most people think 7,500 miles is appropriate, but the N52 can easily handle a longer interval.

By the looks of it I'm not so sure your car has had a decent maintenance plan in it's 107,000 KM life so I agree with ctuna and you should go change every fluid: engine oil, PS, trans, diff, brake fluid, and coolant. Make it easy on yourself and just use BMW coolant with distilled water.

I see where you already pulled the air box. Make sure you didn't lose the little half-dome rubber isolator that sits under the airbox on a threaded stud on the framerail. Sometimes it pulls off the stud and drops into the engine bay. Lots of folks will drop the belly pan and find it and go WTF?

Good luck with the car.