I did oil changes for a friend of mine with a 3.8 powered Grand Prix. He ran Amsoil 5W-30 using 25,000 mile oci's, drove about 50,000 a year. His last change he had 27,000 on oil and EAo filter and he added 1.5 quarts of oil in that entire interval with just under 200,000 mi on the odometer. Only problems engine wise was with the world famous manifold gasket leak, but he got GM to go good for half, even at +/- 100k, IIRC.
Granted that was using synthetic but I think a dino would would be appropriate with the right oci , and especially the Schaeffer's.