The quickest way to determine correct viscosity is with bearing clearance information. Without it is an interative process. With respect to small block based engines anything between 55 psig - 60 psig at 6k rpm is good and anything above 25 psig at idle. That being said the risk of too heavy of an
oil is your
oil pump relieve valve always is relieving most are set at 60 psig from the factory. So if you are below 60 psig you are getting all the flow if you are pegged at 60 psig you most likely are bypassing.
In the end as long as you are not racing anything will work!!!!!!!!