8MM retainers/locks on 5/16 valves: OK or not OK?

I had a good conversation with Mike at B3 racing yesterday & he emphatically stated that the Lunati older tech was absolutely wrong in informing me that my Lunati 8MM locks will be fine on my 5/16 valves. I just now got off the ph with Summit & ordered Manley locks #131518 the only ones he said were available with bead locks as opposed to square groove. I has just noticed that with a valve/retainer/locks in my hand holding them "locked" that I could move the retainer up & down ~.020" so I knew sumpin ain't right. Thanks guys