If the oil has not been changed regularly I would use 30# non detergent as thats what was used in them years ago,if you use detergent oil in a motor that is quite dirty it will loosen up the crud and cause problems. If the motor is really dirty I would suggest pulling the valve and side cover and drop the oil pan and give it a good cleaning and then you can use the modern detergent oils,I have neen using 30# detergent oil in a 57 235 thats in my 37 p/u but it was rebuilt and I know its clean. If its clean 30# detergent oil in the summer will be just fine.