There is also a bunch of actual, measured viscosity range allowed within the SAE's 5W-30 parameters/standards.
I use a 5W-30 in my fully factory stock (not even a tune) car, but, it is one of the 'thinner' ones out there, at a 3.1 HTHS viscosity, whereas some of the 5W-30s have up to a 3.8 HTHSV (like Red Line's).