My guess is that it is now a group 3+/PAO/Ester blend.
Sadly, on most of the 300V Ester Core weights, the VIs and TBNs have gone way down.
2000 Z28 1SC 6 speed 175K miles
Red Line 0W-40/Sustina 0W-20 (20/80 mix)
Fram Ultra filter
Amsoil SVO 75W-140/3.8 oz. XL-3
Red Line D4 in the T56