[Email from Mobil Products: 1. Mobil 1 Tri-Synthetic is a blend of alkylated naphthalene (Group V), PAO (Group IV) and ester (Group V). The alkylated naphthalene is for improved deposit control. The Mobil 1 Tri-Synthetic Formula 15W-50 is A3/B3-B4-98 approved. Mobil Delvac 1 is B4-98, E5-99, SJ and CH-4 approved. Mobil Delvac 1 is a proprietary blend of synthetic base stocks formulated for optimal performance in diesel and gasoline engines. ]
[Comment from Geoff William: The first component (alkylated naphthalene) has the advantage over PAO and esters in that it has the best additive solubility and the best seal compatibility of the 5 most common engine lubricants (PAO, esters (2 types) and mineral oil). This is great news for older cars with brittle seals that might be more suceptible to shinking with a PAO and ester-only based synthetic motor oil. ]