I have nothing aginst running a straight 30 Wt. In the summer, but you have to understand why the engineere who specify the type oil have in mind when they pick a number. Bearing clearances in an engine are chosen for a reason and the tight clearances of a stock engine require a certian viscosity, under certian conditions. Besides lubrication, the oil cools the bearing and a heavy oil does not cool as well as the lighter weight oil. The flathead as a design has lighter bearing loading than a 300hp SBC. The mains and rods have almost the same area and even the best of "Street Flathead" exceed 150 hp and only require 40-50 for cruising around. The BEST part of the design is: It works no mater what you do to it.