There kinda is with our climate. The 5w30 is thinner than a 10w30 when the oil is cold, say after sitting overnight. This allows it to start flowing faster and provide engine protection sooner than a 10w30 on cold mornings. During the summer when the temps are higher even at night the difference is negligible at start up.
Basically, in order to achieve the 2 different thicknesses in a multigrade oil ( a 5w30 behaves like a thin 5 weight at cold temps but like a 30 weight when the engine reaches operating temp) the oil companies have to add polymers to the oil to achieve this flexibility. The larger the spread between the 2 ratings in an oil, the more modifyers they have to add. This is more true with conventional oil than with a good synthetic like Mobil1. Over time those modifyers can cook away inside a hot engine and the oil will 'shear' and lose its viscosity. This happens over time so its usually not much of a problem with 3,000 mile intervals.
Niether do I.......I just post
