I've thrown one quart of ST synthetic in with ST dino. Despite the fact that it's a PZ/QS/Shell owned blending plant, this isn't the same oil as Pennzoil so the warnings are probably applicable. I also think mixing brands for short periods may not be a major problem, I've done it in a pinch.
According to Johnny, ST may switch over to Equilon plants once the Shreveport blending plant is closed. Don't know if it would be identical to Shell or not, or if the Shell and Havoline Equilon oils were identical. Don't even know if it will still be called Equilon once the Havoline production is fully moved to Chevron.
Johnny, if you're there?????