Phillips makes a 25 W 60 multigrade oil for radials. I use it in the 285 hp 9 Cylinder Husoi engine in my Nanchang Cj6A. You get almost instant oil pressure on start up even with cold oil but temps do not get too hot on hot days.
The only downside is it is not recommended for the bigger radials, that is bigger than a 985 because of reports of master rod bearing distress.