No, they are not.
The viscosity testing is set by the SAE. There is no difference between a conventional oil rated SN, and a full synthetic rated SN. The 10W30 is the same viscosity whether it is synthetic or conventional. Synthetic is actually slipperier, for want of a better word, than conventional oil. However all that being said, you use the grade specified by the factory. When both synthetic and conventional oil were being used, there was NO factory manual that ever suggested that a different viscosity oil be used when using synthetic vs. conventional.The factory determines what grade to use, not the oil manufacturer or distributor. The factory designs engine tolerances and clearances, not Amsoil. Redline, or any other oil manufacturer or distributor. When the factory says use 10W30, 5W20, or 10W40, use what they say. We can change those tolerances by rebuilding with tighter or looser tolerances, and you builder should recommend a grade or a brand. that is with full knowledge of what those tolerances are.
Our LA engines were built with tolerances designed for use with 10W30, or 10W40. The Magnums were built with tighter tolerances, so probably use a 5W30. However, using a 0W20 in an LA is taking an unacceptable risk in my mind, not even considering that the tolerances on our older engines have opened up even further.