I have a '98 5.4L - I've seen several comments in several places about the 'door ajar/wd40' issue. While I agree that there's an issue, I think your solution may be only partially correct. When we got the truck, about 3 years and 40,000 miles ago, the 'door ajar' light, the 'ding-ding', the dome light - all harassed us on a regular basis. Most of the time they’d stop before you’d gone too far, but my wife finally took the dome light out, to prevent draining the battery. We’d taken it back to the dealer, squirted wd40 in the door latch – tried ‘em all.
Then – about 18 months ago, preparing for a trip to Texas, she took it to the local “Rapid Lube”, and the guy offered to fix the problem. She laughed and told him to go ahead – knowing the history. He squirted wd40 into the Ignition Switch!! Issue resolved.