At full operating temperature, straight 30 weight oil will be the same viscosity as 0/5/10w-30, so of the two, 20w-50 is "thicker."
For clarification:
The "w" stands for winter and in general, the lower the number, the better the oil will flow at lower temperatures. So a 0w-30 will be less viscous than a 10w-30 of the same brand at the same low temperature. I believe the "low" temperature as described in the ASE test method for cold viscosity is 32F. Lastly, "0 or 5 or 10"w does not mean the oil is a 0, 5 or 10 weight oil at 32F, rather, it means it BEHAVES as a 0, 5 or 10 weight oil would when at that temperature. This is why the lower number offers better protection during cold start-ups as it'll flow better when cold. However, make no mistake that all oils will still be very viscous when cold.
Here's a fun link to read up on that stuff in anyone is interested:
https://www.valvoline.com/about-us/faq/oil-types-weights-and-viscosity-faq
I sure as hell would not want to find out how straight 30 oil flows in my crankcase at 32F (or less, yikes) no matter how worn out my engine is. Before I studied up on multi viscosity oils I once thought it was OK to run 10w-40 conventional in an old I6 powered Ford truck. Well, I went to start that thing when it was -4 or -10 out and it took a solid 6 seconds before the oil pressure gauge read anything and lifters began to clatter around 2 seconds. Poor cylinder walls and main bearings...
