The factory recommendation would have been MS30 in the summer and MS20 in the winter and even MS10 in sub zero. MS with the highest grade of oil made for gasoline engines of the day. ML was light duty, non detergent, MM moderate duty and MS severe duty. Those ratings are meaningless today. My father began selling lubricating oil in the mid 30's and I am still at it today. Many people have different opinions, but I recommend a good 10w30 or 10w40. Any of today's multi grades are so much better than what was available 50 years ago that I think it all boils down to personal opinion.