took the cover off the 91 with 60 miles, 5 were out and 3 were loose.  Those little phillips headed little things.  one fell in the oil passage way, causing an oil leak at the back. 

I had the same thing when I went todo the first (mine and probably the rigs) valve lash job. One had fallen down an oil passage but a magnet probe got it retrieved, another had been bouncing around and had been hit a few times. Definetly NOT what you want going on in an engine than runs at 4K rpm regularly!!!

I suspect that the "valve lash every 15K miles" is as much to tighten those suckers down as it is to keeping the lash adjusted ...