reverse scroll cut in to the crankshaft is intended to force oil back inwards allowing it to drip into the sump. This is fine until any of the following occur: -
The engine is stopped
There is excessive crankcase pressure
The seal is set incorrectly and damaged by the crank. (Can happen with worn bearings too).
Fortunately there is a solution at hand. Revington TR supplies a kit, which includes a more modern rubber lip seal (often referred to as a Gaco seal). See RTR1026K below
Also if a standard part is your choice, we have second hand parts available too