I deleted my prior answer "Its Not". Thats too vague and suggests identical which is not true. I should have said "Both will be within the range of a 20 grade" or "similar weight". This answer includes oils in the same multi-grade. Would take hours of writing to explain why that is, there are many variables. A quick example: TGMO 0w20 is synthetic produced by XOM. M1 0w20 is synthetic produced by XOM. But TGMO 0w20 is not a rebranded M1 0w20, its a unique brew. One of the thinnest oils in the world with one of the highest VI's on the market. Compared to TGMO 0w20, M1 0w20 is thicker with a average VI. So while they are both 0w20's, one is thicker than the other. They could be closer or farther away depending on what operation temperature is. Using the industry standard 100ºC, there is a difference on the warm side but its very small. More of a difference when cold.
Thanks for the explanation. This compares e.g. 5w-20 vs 0w-20 or the same viscosity oil from different brands, right? If I think M1 5w-20 is thick enough then how would M1 0w-20 be not thick enough at operating temperatures? This is for someone who would bash use of 0w-20 but will be perfectly OK with same brand 5w-20 (I see this a lot so I am trying to figure out why).