from an expert on the net..
"...but if it's contaminated by "silicone" then it's resistant to this burn cycle. Or in other words, it's not usable until it's replaced.
It can also become silicone contaminated via the silicone that's used on the circuitry in these sensors because it migrates and then sticks to wire surface.
It can also become contaminated by use of sprays that contain silicone! If you look at MENU#22, you'll see that the 'tire-shine' contains silicone and,
most likely is the cause of my contaminated MAF. As you see from the pictures, the silicone was applied to most 'everything' including the air breather tubes and the air box itself. A very short path from there to the MAF!
The other 'co-contaminate is the filter oil used in the K&N filters. The MAF sensor can also be contaminated from a variety of other sources such as: dirt, oil for vapor, spider webs, potting compound from the sensor itself and the filter oil used in reusable air filters. When a MAF sensor is contaminated, it skews the transfer function such that the sensor overestimates air flow at idle and that causes the fuel system to go rich and it underestimates air flow at high air flows and that causes fuel system to go lean."
and another source
http://www.louv.tv/cars/m5/MAF/