Well, turns out that the "eighth-grade reading level myth" is just that...a myth. Stealing a cue from Greyhawk's post the other day, I decided to run various US Army manuals through the Flesch-Kincaid test. Generally, the test is based on the number of words in each sentence and the number of syllables per word in order to determine reading difficulty. Granted, this isn't the best metric, but for our purposes, it will suffice.
Running a random page from FM 3-24, the Counterinsurgency Field Manual, through the calculator, I discovered that the much-beloved counterinsurgency manual weighs in at a 15th-grade reading level--roughly equivalent to one's junior year in college. The official UH-60 Black Hawk operator's manual (Tech Manual 1-1520-237-10)? The same--15th grade, although I quoted from the systems chapter instead of, say, the emergency procedures chapter, where the words tend to be a little bit shorter. Short words are preferable when the helicopter is burning, to be certain. Oh, and to dispel any rumors out there as to whether or not the infantry has manuals appropriate for five-year olds, just note that the old infantry manual, Field Manual 7-8, clocks in at a 13th-grade reading level.
So what actually does equate to an 8th-grade reading level? Try Reader's Digest, or better yet, the latest State of the Union address.