Multi viscosity oils like 5-20 or 5-30 are using the same base, 5 wt. The viscosity changes by heat coiling or uncoiling little polymer chains. Cold they are balled up, warm to hot they stretch out effectively thicken the oils weight. Point is they are the same base oil and change at unknown temps for the user.
Adding some oil modifier then again changes the oils behavior in unknown ways, could be thicker or not. A user w/o a lab or testing just has no idea what is going on. Any statement of what change to the viscosity change or rate of change is just blowing smoke.
Lighter weight oils do reduce fuel consumption but usually only for brief cold engine operating temps however these lighter oils also are more effective in moving oil at low rpm steady state throttle conditions.
A number of engines also require the lower viscosity to reduce valve train and crank drag to see the design hp/torque. Some engine valve trains can see "coking" because the heavier weights are staying on the valve components for too long of a period versus a lighter weight moving off the components quicker.
Most all of the above is directly related to a stock engine, one buit for performance has other needs and requirements in some instances it is the user/builder to decide how far the build is off oem specs in regards to the critical concerns. Valve train remains the same but at a higher operating rpm level a slightly heavier weight may be called for.
Heavier weight oils with low pressure pumps won't blow a head gasket but could plug by coking off a piston cooling jet though for example.