340 Chugging instead of Reving

I maintain your spark plugs are telling you it looks like one plane of the manifold is sucking oil, with probable vacuum leak.
The other runner, not so much.
That oil is coming from somewhere .
Perhaps put a vacuum guages on it .
My mentor had me read EVERY spark plug I took out, prob nearing a thousand.
They always tell a story .
Hope it helps

THIS^^ I would also do a cylinder compression test.