The WD40 was named for a 1980s tailwater fishery, not the lubricant. It's the simplest emerger in the box — a thread body, a tiny tail of mallard fibers, and a few of the same fibers folded over the thorax as a wing case. That's it. No hackle, no flash, no dubbing.
The minimalism is the point. On hard-fished spring creeks and tailwaters, trout learn to refuse anything that looks "tied." A WD40 reads as a real emerger because there's barely anything there to look fake.
Sizes 18-24 are standard; black, olive, and gray cover midges; baetis blue for BWO hatches. Fish it on light tippet (6X minimum), dead-drift, hanging in the film. Most takes are barely a swirl — strike at anything that hesitates near where the fly should be.