Why are they dead? They sound perfectly good to me. You can't wear out a wire. Strings are dead only when they are covered with oils from your hand. My strings are keep perfectly clean. Very bright sounding.
Here we go hijacking a thread again...
First off, I know that you are the sort of individual that is VERY set in his ways, however, I will reply anyway.
It's not just a matter of wearing out a wire, come on, there's a world of difference between a piece of wire and a guitar string. There is a science to guitar strings, nickel strings (sweet fundamental tones) have a different sound than stainless strings (bright super-defined tone) on an electric for instance. The choice of string (and gauge) not only affects tone, but also how they bend, how they stay in tune, and how they respond to your touch. Round wound vs flat wound also makes a difference in tone, feel and fret/finger wear.
The signs of a set of dead strings may be subtle. Yes, you can get over a year of life out of a set of coated strings like Elixirs (which some folks don't like because they tend to have an almost plastic, muted sound due to their coating) or the high dollar Pyramids, but even then you're pushing it depending on how you play and how often you play. It is a fact that string sets eventually die out, you might not be able to hear it, but if you play as much as you seem to you should know this already. A good sign of dead strings is frequent retuning (no matter how "bright" they sound}. On the high side, strings should be replaced every 3-6 months with normal usage.
It's not only your natural finger oils/acids, it's the way you play the strings, the way you bend them, the delicate surfaces of the nut, frets, and bridge that all combine to shorten the life of a set of strings. Guitar strings stretch, and if they do not stretch evenly you will have a string that will give you artificial harmonics and is difficult to keep in tune, a sure sign that it's time to replace the strings. If I remember correctly, didn't you post a photo of your guitar somewhere in this forum? Does it have a tremelo bar? A whammy bar is deadly to string life.
WD40? Never would I put that anywhere near my guitar. Yes, I know, some folks like it, but you can find some folks that will like anything. Earl Scruggs used to put hair tonic on his strings before playing, but that doesn't mean I would. Ever painted cars? A tiny bit of WD40 in the air from somewhere nearby while you're painting will royally screw up a paint job, most concientious painters will not allow a container of WD40 in their shop.
Okay, with that said, I know you'll continue to put WD40 on your strings because it works for you. And you will likely go down in the Guiness Book of World Records for going the longest without changing a set of strings on your guitar.
New strings don't cost much, five bucks for a decent set. Regardless of how you use it, a guitar is an instrument and should be treated as such (don't get me started on bands that smash up their equipment).