Also check the PCV system.  Under the intake manifold at the front
(belt) end of the engine is a black plastic box that is the oil trap.
With poor oil change history this clogs up, as do all the associated
hoses.  This box is impossible to clean properly, and you need to remove
the intake manifold to replace it.  Under the throttle pulley cover is
some of the plumbing associated with this system including the flame
trap.  Start by looking at it, and the hoses that are easily removed
around it.  When this system is clogged up, the blow by pressure looks
for other ways out of the engine, often causing leaks at various seals,
popping the dipstick out, etc.