The hot wire mafs use ceramic elements. Once
oil gets into the pores of the ceramic and the meter is powered, it turns to carbon and those deposits are almost impossible to remove from the pourous ceramic. My guess is that if your meter has suffered from a lot of
oil contamination, the calibration has shifted significantly.
Example. An over oiled K&N filter on a 5.0L mustang will do this...and the result is a meter that reads about 10-15% lean, even after cleaning. Seen this more than once; and just wrote code to compensate after using the dyno and afr to tell me what the offset was.
The frequency to voltage conversion stuff is how Massflow uses the GM/Delphi style meter on a Ford system. The native output from that style maf is frequency, and they use a conversion box to convert the signal back to analog so the Ford processor can read it.
Byron