The 1st number is what you get when you start the car, it contributes to cranking speed. It's not relevant if you live on Mercury or Venus, otherwise I'm guessing that your air temperature is below 180 degrees.
A missing (no ?W) or high 1st number (20W & up) is inverse to how many seconds it takes before the last bearing gets oil. Fewer seconds is better.
If you use "straight" 30 weight (W is winter, not weight) I suggest you follow the procedure they used in 1916 aircraft: heat the oil over a stove, and add it to the engine just before you start it.
Really modern engines can live with 0W and 5W, we can't. Don't.

Over the decades, many smart people (Norton Player team, Gene Berg) re-discovered than running asphalt in the pan breaks things faster than the original viscosity.