I do not know GMC engines. Though on top of my Chevy stovebolt the valve spring there is a metal cap. Looks like a small hat with a very wide brim that sits on top of the valve stem end and covers the top of the valve spring. The rocker arm pushes down on this "hat" to open the valve.

This "hat" is the valve spring rotator. My first thought when guessing what it was called an oil shield just as you did.

The valve spring keepers and valve spring retainer sit underneath the valve spring rotator.

I think Kanter Auto Parts sells the valve spring rotator with a new spring. Jeffrey