A little filter oil shouldn't damage the MAF, but it may throw the readings off. If it does, the car will run like a bag of poo. If you are worried about it, pick up a spray can of electrical contact cleaner. If you have a problem, the sensor is easy to remove and spray out. That will fix it if it is fouled by oil. The proper oiling procedure depends if you are using a squeeze bottle or a spray can/bottle...
http://www.knfilters.com/cleaning.htm