It probably doesn't have anything to do with the oil pressure sender. At most, it may be a loose connection that gets jarred when your engine stumbles.

When your fan turns on, the load on your alternator increases suddenly, and it becomes harder to turn. The engine stumbles and loses RPM for a moment before the idle control valve lets in more air. When your RPM's drop, your oil pressure also drops for that moment. So the light flashes. Nothing to do with the sender or filter.

Now, you have to fix the stumble. Clean all your grounds so your electrical stuff is good. Do a throttle body cleaning, and clean the idle air control valve. Instructions are in the 700/900 FAQ. This should almost/completely eliminate the stumble. If not, you have some other additional minor problems.

Greg