Hello I will help you with your question,
Most cranks are cast steel but some are forged.
Let me know what I can do to help you with this
If you can provide the context I may be able to help but when you say "What is the "main metal" of a crankshaft"? I hear you asking "What is a crankshaft made of"?
Please provide the context of your question and I will help you with the meaning.
I think they mean the main bearings of the crank. I have worked on Japanese and Korean vehicles for more than 20 years and some of the translations can be obscure when you read them in english but make perfect sense in the native language.
It would male sense to add additional lubrication this way but it would reduce the horsepower available if the oil wound up around the crank.
Many engines use a "windage tray" that almost touches the crank to remove any oil that winds around the crank so it is available to lubricate and to reduce the power required to move the crank and the weight of the oil.
Is this a road vehicle such as a Prius/Insight or something else?
I understand and having the "main metal" gravity lubricated would make perfect sense to keep some oil on the bearing
Let me know if there is anything I can do to help you with this