As you know, Sunoco is a major partner and supplier in motor racing, particularly NASCAR where Menards is a sponsor of Team Penske.
1956 introduced "custom blending" at Sunoco stations and the "Sunoco Blue" logo was the distinctive symbol of this new product feature.
"Sun revolutionized the oil industry when it introduced the Custom Blending Pump, a novel system for dispensing a choice of five octane grades of gasoline from a single pump. A model of the pump is currently on display at the Smithsonian." - Sunoco Website
The box truck could be delivering the new pumps to Sun Oil Company service stations around the country.