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

A “bad fit” for a single groove valve is any combination of parts that won’t lock together.

When the parts aren’t locked together, and the tangs are bearing the load, every time the valve closes there’s a little impact load on the tangs when the valve stops moving.
At higher rpms, especially if the valvetrain isn’t in 100% complete control from the spring(which is much of the time), then the tangs on the locks are literally getting hammered every time the valve slams into the seat...... and that’s just what they look like is happening.

When the interfit is correct, the higher the spring pressure, the harder the locks squeeze the valve, relying less and less on the tangs.
This is the rationale behind the “super 7” and “super 8” locks.
A thicker cross section 7 or 8 degree lock to add more area to combat retainer pull through, but utilizing the steeper(than 10*) angle which does a better job of wedging down into the retrainer and squeezing the valve even harder than their 10* counterparts.