The weight of the oil is not really the issue.
You do not want to use a motor oil rated as API "SM" in an engine with flat tappet lifters. Valvoline VR1 comes in two different versions one that is "SM" rated for use in street vehicles and another that is intended or "off road use only". the difference is easy to tell, the "SM" rated oil is $2.50 -$3.00 a quart and the real racing oil is $4-5 a quart. I checked at my local auto parts store and found VR1 in various weights all of which were "SM" oils , so they were "WORTHLESS" for my needs!!!
The major difference is the levels of ZDDP additive in the oils. The "SM" rated oils have ZDDP levels below 1200 ppm which will cause significant camshaft/lifter wear in engines with flat tappet camshafts. The "for off road use only" real racing oil has the proper levels of ZDDP that flat tappet camshaft engines need.
just my $.02
Bob