Tommy,
1959 Chevrolet Truck Operator's Manual - Lubrication GM's specification/recommendation of multi-viscosity oil started with 1954 engines (single grade oil was also specified/recommended). Of course, the benefits of multi-viscosity oils were the combination of the "easy starting characteristics and . . warm weather operating characteristics".
Nonetheless, if you want 30W oil, have you checked NAPA and Walmart (they have both detergent and non-detergent 30W oil in our area)?
