one
wonders why Continental doesn't use them, since their barrel style
hydraulic lifters provide substantial oil for additional cooling of
the
guide and valve. Lycoming's mushroom style lifters do not.

Lycoming uses hydraulic lifters in all but the O-235 series engines,
and whether hydraulic or not, they send oil up the pushrod to the
rocker and valve stems. They are located above the rocker shafts and
valves, and get plenty of oil on them. Lycoming's camshaft is above the
crank and tends to get little cam lube at idle. Continentals have their
cams at the bottom, getting lots of oil, but the valves are high and
get a bit less drip. They have a gallery thru the rocker to make sure
some oil gets to the rocker tip and valve stem.
Lycomings problems are not so different from Continentals. I've had
much more valve trouble in the small Continentals than in the Lycs, and
what valve trouble Lycs give is due more to the guide material (appears
to be bronze), which doesn't have really good wearability at high
temps. First sign of trouble is a leaky exhaust valve, as the guide is
allowing it to seat a bit off-center and wearing one edge.

Dan