Shell is a reputable company. I wouldn't dismiss this as a marketing gimmick, even though marketing is always a factor.
Looking at the TDS's, the difference between Pennzoil Platinum (PP) "Gas Automobile" and Shell "Gas Truck" is viscosity and/or viscosity index. Gas Truck 5W-30 has KV100 = 11.7 cSt as opposed to the Gas Automobile (PP) with KV100 = 9.8 cSt.
With the xW-20 viscosities, the difference isn't that obvious but if you look more closely and go ahead and calculate the VI, VI = 166 is less for Gas Truck 0W-20 than Gas Automobile 0W-20 (VI=182) and similarly 156 and 167 for the 5W-20's, meaning less viscosity-index improver (VII) used in the Gas Truck than in the PP.
Therefore, Shell Gas Truck has a thicker oil and/or less VII than PP, which means that the oil film will be thicker with new oil and/or the oil won't shear as much with used oil, resulting in thicker oil film with used oil. Therefore, there is a validity to Shell's claim that the Gas Truck oil will protect the truck engines, which have higher loads, better than regular oil, as the Gas Truck oil is a little thicker and/or doesn't shear as much as their Gas Automobile oil (PP).
Labeling the oil as "all-mileage" is also consistent with this observation, as high-mileage oils tend to be thicker.