Ideally, yes, all 30 oils should have the same viscosity at operating temperatures. But practically, a 10w30 will be slightly thicker than a 5w30, but within the SAE margins of a 30 grade oil, keeping everything else the same. Like, if you check the viscosity of same oil brand, same specs, 5w30 has, let's say, 10.1 cst viscosity at 100 degrees, a 10w30 will have a viscosity of 10.2 cst. Both are within the specs of 30 grade oil, which is between 9.3 and 12.5 cst, as mentioned by SAE.
For example, a Kixx G1, 0w30 has a viscosity of 9.74 cst at 100 degrees. while a 5w30 has a viscosity of 10.13 cst at 100 degrees.
