What is proper oil grade for chevrelot 350 motor?

30 - weight, or 10w30, if you live in an average climate and have a < 400 HP. Colder climates may require 10w30, and very hot climates or high-performance or heavy-use engine (i.e. boats) may need higher viscosity (10w40 or 20w50). Synthetic oils can maintain viscosity better in "hotter" engines, and is highly recommended for them!