I've used a spray called "Lock-ease" that is similar to WD-40 but contains graphite also. The lubricant in WD-40 is notoriously short-lived. It's really more of a water displacer and penetrating lubricant than a long-lasting high-quality lubricant. I'd be less afraid of Lock-ease than using an actual oil in my lock cylinders.
If the key isn't going in, I suspect that the lock tumblers are jammed or otherwise messed up.
The dealer can order you a new door handle for about $120 and you get a new key. Give them your VIN.
My 1983 SD needed the driver's door done, so it was less a matter of chivalry and more of a pain in the butt to get into the car.
Hope this helps,
Lehigh Valley PA USA
73 Olds 88, 72 MB 280SE, 78 Datsun 280Z, 71 T-Bird, 72 Olds 88, 83 Nissan Sentra, 85 Sentra, 73 230.6, 91 Integra, 83 300SD, 91 Volvo 940GLE wagon, 84 300SD, 95 Subaru Legacy L wagon, 02 Mountaineer, 91 300TE, 08 Murano, 2007 R320CDI 4Matic 52K, some Hyundai, 2008 BMW 535xi wagon all gone... currently
2007 Honda Odyssey Touring, 2011 Honda Odyssey Touring