I have used rotella 15w40 from day one. mainly because I run that in my daily driver which is also a gas burner and i want to keep the diffrent oils i stock to a minimum. I also run the rotella in my 4020 john dear, model D john deere, and 600 ford tractor. The flathead does not have real high spring pressure so zddp aditives are not that big of a deal as in an overhead valve engine with .400"+ lift. A flathead requires a lot less spring pressure than an OHV. However valvoline race oils still contain good amounts of zddp as they are intinded for off road applications. But any good quality oil will be fine. a dual viscosity oil was not available during the flatheads hey day but is a good choice because it flows better at lower temps than a straight weight oil.