The principal structures identified are shown below (in decreasing order of concentration): ##STR1##wherein R is the long, branched alkyl chain and comprises about 60 mole % (C.sub.4).sub.4 to 35, about 30 mole % (C.sub.4).sub.12 to 35 and about 10 mole % (C.sub.4)> .sub.35 ; R' is mainly methyl but is also ethyl; and the ratio of iso-C.sub.4 ton-C.sub.4 is about 3:1.With respect to polybutene add