"SJ being the current one for gasoline passenger cars"
How old is the article referenced in that thread?
Formulations change, so even if M1 contained no Ester at some point, that isn't necessarily true.
Everything I have ever read indicates that M1 is a blend of mostly group 3, but some group 4 and 5. I've never seen anything saying it was only group 3, and that would surprise me being that Mobil is the largest producer of synthetic base stocks.
Edit, ok, so group 5 products that aren't esters.
"ExxonMobil Chemical offers innovative Group V alkylated naphthalenes under the trade name Synesstic(TM) blendstocks. These novel, highly stable Group V
blendstocks are formed by the reaction of olefins with naphthalene.
"Alkylated naphthalenes offer the hydrolytic stability of a PAO and excellent additive solubility of an ester They also can help improve additive system efficacy
because they are less polar than esters. Lubricants blended with Synesstic(TM) AN products provide increased hydrolytic and thermal/oxidative stability, resulting
in increased oil service and longer equipment life."