WD-40 won't help. The hinge isn't even visible unless you remove the aluminum cover (held in place by two torx screws). The hinge is actually a steel pin fitted inside a metal (aluminum I think) cylinder which seems impervious to lubricant. One of these broke on my 867 MHz PBook last year and I eventually replaced it. Powerbookmedic.com sells the hinges, as do others.
If the hinge seems to be getting tighter over time, be careful opening/closing the lid. I'd suspect a broken hinge is not far away.