My 5 series Tek appears to use a very typical 45w Howard Industries fan. These are nothing but glorified refrigerator evaporator fans, and equivalent models are still commonly available.
It looks to me like it would be easy to up the motor wattage a notch while staying in the same frame size. One could then pitch the OEM aluminum fan blades a little steeper by hand (not that hard to do - I've done it on other fans) and maybe tighten up the dimension of the fan flange with some plastic trim or something similar. (That's where the efficiency of these fans really suffers, and the tighter the fit, the better the efficiency. Of course, if the bushings get loose, a tight tolerance fan will hit, but hey...that's why you're supposed to replace those bushings when they "become worn" just as it says in the manual. :-)
FWIW, if anyone is experiencing inadequate airflow, IMO it would be pretty easy to gain 15-20% airflow increases while keeping an original look by doing these or similar steps.
Filter oil - yeah, ditto on the K&N oil. Works great as long as it lasts, which is quite a while in reasonably clean environments.
Finally, similar shredded wheat metal filter material is available from restaurant supply houses. (Grease traps use a similar material) and the C channel that makes up the filter frame is common as well. This, just in case someone wants to fabricate a "near correct" filter for their 5 series. FWIW