Typical for the old Dubuques. If you can find it 140wt. I would suggest the heaviest gear oil you can find or 60wt motor oil. Add two quarts of Lucas oil stabilizer first and then finish with the oil. The final drives need the same thing.
My M had the same high speed whine but mine was mostly in the right final drive. I used 2 quarts of the Lucas oil stabilizer and finished filling the tyranny with 140w gear oil. I actually filled both final drives with Lucas hub oil as there's no brass in them. It was costly but there's no noise now. Your results may vary.