RE: 5w30 vs 10w30
I use valvoline 10w40, gm just wants you to use the 5w30 so their mileage rating are better, same reason they cut the skirts on pistons for higher mileage.
Yet now gm engines have piston slap.
friend has used 20w40 in his 350 engine and has 314K miles. so it must be good.