Re: Shell Helix Plus Semi
A 10w-40 is a 10w-40, there should be no discernable difference really.
It may be that the detergents in the Shell oil are better and therefore have dislodged some built up deposits causing the tolerances to widen and therefore some additional rattles.
Magnatec is not the dogs goolies of oils, the basestocks are fairly low grade. The Shell may in fact be a better quality oil.
They're all petroleum based oils, if you want a better quality oil then use a proper (pao/ester) fully synthetic 10w-40 or 10w-50 and you'll notice the difference.