No what I am saying is that the multi viscosity ratings of modern oil lists the viscosity of 0 degrees C as the first number. Think about it, Ford uses 5W20 in the majority of their newer engines, they do it because it is thinner at operating temps and therefore they get a higher CAFE rating. They use 5W30 in the Ecoboost motors because of the turbochargers and the oil is thicker at operating temps. Same reason why the new GT500's and BOSS 302's take Motorcraft 5W50 for the increased protection at operating temp.
If you don't believe me that is fine, but you should educate yourself on oil analysis and understand how the SAE oil viscosity ratings work. Also if you don;t believe me, rest assured because EVERY oil manufacturer agrees with me, because that is the information they use.
Back on topic, either weight is fine really. The literal only differenceis the viscosity when cold between a 5W30 and 10W30. On a pushrod motor that is not as critical as a OHC motor.