One aspect of the oil supply to the top end of the engine which seems to be overlooked is the oil distribution to the spindles on which the cam, double gear &, in the case of a gear drive engine, the intermediate gear rotate.
Oil is fed from the oil pump via the steel tube cast into the timing cover to the end of the crank. A restrictor on the crank end diverts excess oil up the tube to the aforementioned spindles which are fed via drillings in the casing which pierce the supply tube. Oil flows into the spindle ends & out, via cross drillings in the spindles to the rotating gears/cams/chain sprockets.
Oil not fed to the spindles is fed, via a restricted union, to the rockers. The familiar T-shaped oil feed pipe has 3 x 8mm banT-shaped each of which has a restrictor drilling of approximately 1mm diameter.
The function of the restricted banjo bolts is a matter for discussion. In a world where oil pressure & flow from the pump was high they would indeed restrict the flow to the rocker gear to an adequate level. In the real world we have low pressure & flow. In this case the restrictors actually prevent the oil from taking the route of least resistance & in doing so prevent the crank & cam from being starved of oil.