As dfaris said, and I'd suggest using a "penetrating" product such as "Liquid Wrench", "Kroil" or WD-40 instead of a lubricant. The solvent 'vehicle' in the penetrating products help break up any corrosion in the threads and dissolve dried/gummed old lube or thread locking compound that might be in there.
I'd also ditto that the screwdriver tip must fit the slot precisely in both length and width for best results. That little screw is very easy to booger-up otherwise.
The blades on standard "hardware store" screwdrivers are usually taper-ground. This precludes their getting complete, positive engagement to the full depth of the slot. "Gunsmith" bits are hollow-ground, making the sides of the blade parallel to each other and ensuring that the force applied is evenly distributed over the entire surface area available.
There's another small 'trick' that might help when used in conjunction with the above:
After allowing a couple of hours for the penetrating product to do its work, secure the receiver against inadvertent movement as much as possible, preferably with the side with the fastener you need to work on horizontal.
Insert your perfectly fitting screwdriver blade into the slot holding it perpendicular and firmly planted against the bottom.
While continuing firm pressure downwards, start applying just enough torque counter clockwise to place some rotational load against the resistance.
As you maintain that torque, give the end of the driver a series of light taps with a small hammer. In most cases this will result in getting the fastener to break loose and start moving.
