PURPOSE: To obtain a group II metal overbased sulfided alkylphenate composition having low viscosity at high TBN.

CONSTITUTION: There is provided a group II metal overbased sulfided alkylphenate composition derived from an alkyl phenol rich in alkyl substituent bonded to a phenolic ring in a middle position, represented by formula I: (wherein -CRR'R" alkyl substituent is substantially linear; further, R" is H, methyl, or ethyl; and R and R' are each an alkyl having at least three carbon atoms, provided that the total number of carbon atoms of R, R' and R" is 11-21, the difference between the number of carbon atoms of R and that of R' is 3 or smaller; and (n) is 1-2).