With the external lead back to the cases, the oil cools more than in the hot cylinder.
You can tap it in at a level BELOW the highest oil level, since when running the engine, that drops . Not really convinced that crankcase pressure pushes it back or holds it in the head.
Kevin did the rear cylinderhead with an external, following NCR, to give an extra oilfeed to the secondary gearboxshaft.
As this is only lubed by oil spinning of the crank (and from the typically removed kickstartgear) and was sometimes stressed to the limits, every help was needed. A hole was drilled from the top of the cases into the place where the bearing was, and the shaft was drilled inside with some small holes to feed the metal gear/shaft bearing surfaces.
Another reason was that a good pump would suck all the oil out of the cases when running for too long at max revs, ending some engines with rod problems. So the faster the oil went back, the better, hence the special covers inside to slow down the "mist" and get it back down in the cases.
As some tuners found out, the mist can slow down the crank at high revs, dropping the power.
Lowering the oil level would increase hp, but on a testbench there is VERY little acceleration, braking and turning going on, so the oil flows back rather at the case and the level stays rather consistent, instead of being smashed against all sides.
Hence the very deep cases as used in later engines, less mist cause the oil is far away from the crank.
Dry cases anyone ?

ciao
ducadini