With all the AD's for things with a very low likelihood of occurrence, you
really have to wonder why this (from Cessna Pilot's Association) isn't one.
It should be and you should check your engine immediately.

Many engines, particularly Lycomings, have a pressurized oil passage opening
into the flange for the vacuum pump mount. This was to supply oil to a
corresponding hole in the old wet pumps. With a dry pump, the hole is
simply blanked off by the gasket. The pump manufacturer sometimes supplies
a cork gasket or one of cork and one of rubber. If a cork gasket is used,
the only thing preventing the oil from being pumped out of the engine under
oil pump pressure is a 1/8" inch section of 1/6" cork. Not exactly what you
would bet your airplane or your life on. A number of planes have already
invented new landing fields when this little smidgen of cork let go.

Look carefully where your vacuum pump mates to the engine. If you see cork,
it should be replaced immediately.
--
Roger Long