Not the same, but close. Every oil will have its own viscosity, at each temp, measured in centistokes.
So when you say startup, what temp are you talking about? Because in Alaska, the cold start winter viscosity is going to be drastically different than the cold start viscosity in Phoenix in the summer.
One paper I read showed when oil is freezing solid, right below its pour point, it has about 100,000 centistokes. This is about -45 degrees C. At operating temp, 100 degrees C, a synthetic 5W20 has about 9 centistokes.
So a synthetic 5W20 has between 9 centistokes at operating temp, and 100,000 centistokes when the oil is frozen solid and almost pours.
The 5W30 vs 5W20 labeling is a layman's labeling system, to dumb down what the oil does for the average consumer. Most folks wouldn't understand what viscosity oil to buy if it was labeled by centistokes.