In a 50 grade oil, SL does not automatically mean better. I use Mobil 1 15W-50, SM, and it has 1200 ZDDP.
Look for the fine print on the specks. If it says it meets the emission equipment protection of ILSAC GF-4, then it is the low ZDDP levels.
I was at O'Rileys yesterday, and they had found some cases of old Valvoline full synthetic MaxLife, in the red bottles. The date code said it was made in april, 2004. It was rated SL. You would be surprised what you find in "back rooms, storage rooms" in some places.