Three things, really, I would guess -- reduced wear on cam follower and
adjuster nut (also reduced wear on the cam lobe, but that won't affect
clearance), reduced wear on guides (and hence better seating and
reduced wear on seat), and less ash in the combustion chamber since
there is less consumption and less zinc stearate in the oil (I think,
not sure on the last one).
I've not run Mobil 1 in a 61x engine long enough to need to do a valve
adjust yet (will be checking my brother's 300D this weekend or next),
but on the Volvo, only a couple valves were out of spec after 50,000
miles.
Peter