If you want to follow the original owners manual ask yourself why GM changed Dexos 1 by two generations since your manual was printed and used the synthetic blend...do any of the updates affect your engine???...would using Dexos 1 Gen 2 or Dexos 1 Gen 3 make a difference???...does any conventional oil meet your owners manual recommendation???
my 2005 GMC Yukon Denali 6.0 was made before Dexos was a thing at GM but I still prefer to use a Dexos 1 Gen 3 (now) full synthetic oil at least partially for the reasons on the chart shown by 4WD above my post...and partially as two other vehicles I maintain also use 5W-30 Dexos 1 Gen 3 oils per GM recommendations...
Bill