I'm trying to adjust the feed dogs on my treadle. Here's what the manual says: "To raise and lower the feed bar, turn the machine head back. On the end of the feed lever will be found two adjusting screws, loosen the small one on the upper side of the lever, then turn the large screw to the right to raise feed, to the left to lower the feed, then tighten the small screw." I've pretty sure I've found the screws, but I can't turn them. My big question is whether it would be safe to try WD-40 on them?