Nope. I'm absolutely not kidding.  Nor am I ignorant, misinformed, or otherwise challenged.
Fact is, wood is dead. When dead stuff is subjected to heated air, it loses its natural oils, which causes shrinkage. Shrinkage causes stuff to crack, due to uneven tensions within the piece. Some woods are more prone to shrinkage/cracking, including rosewood, walnut, ebony, purpleheart, etc...
By reintroducing a natural oil, the surface becomes sealed, and slows this process. Lemon oil is one traditional substance that has been used for centuries- I simply don't care for it. It is too thin, and evaporates quickly, requiring more frequent application. Olive oil is thicker, thus it is absorbed more slowly and lasts much longer.
Lemon juice is acidic, and will do more harm than good. Really bad idea. 3 in 1 oil is a petroleum distillate, which will actually work, but I would avoid it in favor of a natural, non toxic product.
I have seen fretboards split, cracked, and buckled due to drying out, and it's a shame. You were lucky with your instruments, now you are informed.  
