Design and clearances dictate what viscosity
oil you should run.
Putting 40W in an over head cam engine in cold weather can wipe out the cam and head, as it takes too long to get the thick stuff pumped up there. on start up.
Most performance pushrod engines should have thicker
oil, but I have to wonder how many distributor gears were lost from running to many rpm on a cold engine with thicker
oil than necessary.