best type of oil for 71 340 engine

What suddenly calls your attention to oil weight?

If you google oil viscosity you'll find it starts based on climate/temperatures.

Start with the recommended weight according to your climate, then based on bearing clearances 'if you know them' use your best judgment to determine wether you need more viscosity.
For ex. typical california car never see's/operates below 20*, so no need to run less than a 20 wt , now in the summer it can get into the 90's-100's in most of the state..so more viscosity-50wt=20/50wt oil, however if you spend most/all of the driving in the cold parts, 10/40wt is fine.

jmo