Well, I guess tightening up the through-bolts is OK, as long as you
don't over-torque them.

I just rebuilt an O-300 and have worked on several of them. They are
great engines, though at 145HP and 7 to 1 compression not behemoths of
power. Crush washers on the rear case and on the oil sump are bad to
leak, if they have been over-used. Buy new ones; they're cheap anyhow.
Sometimes you'll have a leak on the rear case where a stud has been
pulled loose from the magnesium housing, like at the seal cover for the
tach drive.

I see a lot of leaks around the pushrod-tube-to-crankcase boots, which
are easy to get off their seats or not have their spring clamps seated
properly. Those boots are tough and will last to TBO, but sometimes
you'll have to adjust one to get it to do its work.

To find a leak I always clean the engine and fire it up with the cowl
off. Run it just long enough to get the oil warm and don't fry your
cylinders. Leave your aircraft on the ramp so you can see where it's
dripping. Invariably we have found our culprit leak and been able to
stop it.

I always paint gaskets like valve cover gaskets, and all the other
gaskets, with High-Tack, front and back. If you get any leak around
any of those gaskets the oil will be dyed red by the High-Tack, so you
know generally where it's coming from.

Of course, CA glue is great for stopping seepage around bolts and seams
in the case. Put a vacuum cleaner to the oil filler and apply the
glue.

Finally, if people would handle cylinders gingerly and not hold them by
their pushrod tubes, the damn things wouldn't leak. Having to use
that beading tool to seal the tubes inside the cylinder head just means
the cylinder was abused by someone.