Here is the relevant SB for the Rocker box vent
https://jabiru.net.au/wp-content/uploads/2018/05/JSB013-1.pdf
Take note of section 3, copied below
Jabiru Service Bulletin: Jabiru 2200 Engine Rocker Chamber Vent
JSB 013-1 8th August 2006
JSB013-1.doc Page 2 of 5
3 Background
The upper valve train of Jabiru engines is lubricated by means of a low-pressure feed which
supplies oil to the valve guides and rockers, then flows back into the sump via the pushrod
tubes.
In the engines listed above the flow of oil back into the sump can be reduced by internal airflow
and pressures inside the crankcase. This effect is magnified by a build up of negative pressure
inside the rocker cavity (as air is drawn out of the rocker cavity by the normal leakage past the
intake valve guide) and by the additional oil flow from the hydraulic lifters.
This restriction in the amount of oil flowing back into the sump leads to the rocker cavity filling
with oil. As the rocker cavity is at a high temperature (typically over 130° Celsius) the oil is
overheated, becoming very thin and gradually loosing it’s lubricating properties. This leads to
increased upper valve train wear (valve guides, rocker bushes etc) and elevated oil
consumption as the thin oil is sucked down the intake valve guides and burnt in the engine. In
some cases oil may also be found in the induction manifold.
This phenomenon varies from engine to engine due to differences in operating regimes,
different average oil levels in the sump, different engine vent outlet pipe positions and different
levels of blow-by past the piston rings. Jabiru 3300 and 5100 engines (including 3300 engines
with hydraulic lifters) do not suffer from this problem due to the larger airspace volumes in their
crankcases and oil sumps.
Drilling a small vent hole inside the rocker cavity removes the negative pressure inside the
rocker cavity and allows the oil to drain normally into the sump (Note that as the pressure inside
the cavities is generally less than ambient, air is sucked in through the vents – only negligible
amounts of oil flow out). The following Service Bulletin details the installation of the rocker
cavity vent and ways to manage oil consumption before and after the vent is added