Some engines oil thru the
head itself and the oil goes down a tube that mounts the rocker arms, usually. Some engines like the GM V8's oil thru the push rod tubes as they are individually mounted on studs, the oil goes thru the tubes and out a hole in the rocker arm, since the rocker arm is rocking around, it will make quite a mess. Slopper stoppers were nothing more than a short piece of wire with a head on it, you put them into the hole in the rocker arm and it stopped excess oil, never tried it, but bet a cotter pin would
work.
I have never heard of adjusting solid lifters when running, old hot rod trick was to run the snot out of an engine and get it as hot as possible, then quickly adjust the lash to zero clearance, theory was to maximize the
lift that way, surely a pointless endeavor as a few thou one way or the other won't make much difference, and for our Diesels would be pure folly.
I'd be interested in hearing how you adjust solids when running, the GM rocker arms had a big nut in the middle so that it didn't move with the rocker arm, the adjusting nut like on a
Yanmar would be tough to adjust when running as it's moving with the rocker of course.