Cont O-300D oil leak problem

Small Continentals typically leak from both ends of the pushrod
tubes. The rubber couplings at the case need to be replaced, which
requires pulling the cylinders off, at which time the cylinder base
seals (o-rings) should be done, too. The head end of the tubes will
often leak until the rocker covers are removed, cleaning solvent (like
Brake and Parts Cleaner or laquer thinner) is used to flush out the
tube/head joints, and a wicking Loctite is run into them. If it's the
front crankshaft seal, there's a split version available to install
without any fuss.
The oil tank or accessory cover gaskets might be leaking, and
worst of all, the crankcase joint might be starting to seep.

Dan