OK,, I was assuming ,,, which is always dangerous...
Is your blower motor actually running slower... or are you saying that you get less and less air from the vent.... if the latter then I think Rick may be correct....
For oiling I would stick with something like Singer Sewing machine oil... or get one of those special oiler pen sized deals...I think several people make them ... with Mobile one, or such special lubricants... you do not want Automobile oil for that.... If you want to use the Marvel mystery oil I would only use their ' air tool ' oil for this application...