Let’s clarify a few things. The first number is the winter rating, which is what the “W” stands for, and an oils winter rating is determined by how it performs in two tests. The first is the cold cranking simulator (CCS), which measures the ability of the oil to allow the engine to crank at cold temperature, and the second is the mini-rotary viscometer (MRV), which measures the oils ability to be pumped at cold temperature. In the CCS test a 0W oil cannot exceed a viscosity of 6200 cP at -35C and the 5W cannot exceed 6600 cP at -30C, while the 0W cannot exceed 60,000cP at -40C and the 5W cannot exceed 60,000cP at -35C in the MRV test.
The second number is the oils kinematic viscosity at 100C (212F), which is considered the normal operating temperature of an engine. Each grade is actually a range of viscosities, not a single number. For example a 20 grade oil can be anywhere between a minimum viscosity of 6.9 cSt and a maximum of 9.29 cSt with a minimum HTHS of 2.6 cP, while the 30 grade range is from 9.3 cSt to 12.49 cSt with a minimum HTHS of 2.9 cP. The difference between the minimum HTHS of 20 and 30 grade oils is only .3 cP, and there’s no denying that a slightly higher HTHS is going to provide more protection, which may be important in high engine stress situations.
You used the terms “tolerances” and “tolerance” when you’re actually referring to an engines clearances. Clearance is the distance between the moving parts and tolerance is the amount of allowed variance in the clearance dimensions Engine clearances haven’t changed much in the last several decades but tolerances have improved with modern production methods. The idea that the clearances in the 5.7L engine are too "tight" for the slightly thicker 30 grade is simply not accurate. During a cold start 5W-20 is going to be many times thicker than a 5W-30 is at operating temperature, and will remain so until it’s fully warmed up. Also, the difference in KV in most API rated oils is fairly small between 20 and 30 grades. For example, the KV of Pennzoil Platinum 5W-20 is 8.6 cSt while the 5W-30 is 9.8 cSt. At 40C (104F) the KV of the 5W-20 is approximately 46 cSt, which is 4.5 times thicker than the 5W-30 is at 212F. If a fully warmed up 5W-30 is too thick to fit through the “tight” clearances of the engine then how can a much thicker cold 5W-20 do it?
The 5.7L will function perfectly using a 30 grade oil that’s only a couple cSt thicker than the recommended 20 grade, it's up to each owner to make an informed decision whether or not he or she wants to go outside the recommended, not required, 5W-20 in their 5.7.
Here is SAE J300 which shows all of the grades viscosity information.