If I park my car in a garage can I get away with using a thicker oil? Say a 5-30 instead of a 5-20? Since it would never start cold?
Starting your car in your garage constitutes less than half your startups, given shopping trips, parking at work, etc. So you can't really say it would "never start cold".
However, you're asking about 5W-30 and 5W-20, both oils have aprrox the same cold viscosity.