Tommy, the original grub screw is a slotted one and generally sits level with or slightly recessed in from the bottom face where it screws into. Make sure the slot is cleaned of dirt and crud, and you are probably best to use an "Impact screw driver" and make one of the blade tips as good a fit in the slot as you can, then with a couple of good "Blows" with a hammer it should come loose. It is usually about 3/8 ths of an inch long, but the cross drilling is higher in the crank case than this. It is hard to say what if any could restrict the flow, but if something from the scavenge trough in the back of the case was big enough, it could happen. You might need to remove the case sump plug and try to flush some kerosine or similar into this hole and see how it flows out the other side once you have the small grub screw removed, and you can also clean up this hole with a small bottle brush or similar. You might be able to flush using some rubber pipe up against the hole and back flush the other way to wash away any restriction. The grub screw would be better replaced with a new one, make sure the hole is clean and free of oil and kero, and wipe a smear of thread sealant on it, then screw it home fully. Cheers and good luck.............Greg.
