A synthetic sulfurized oil, useful as an additive to lubricants, and especially as a replacement for sulfurized sperm oil, is produced by reaction of sulfur, lard oil and polyisobutylene oligimers containing at least one pair of maximally crowded geminal methyl groups (e.g. "tetraisobutylene"). Preferably, sufficient sulfur to obtain a final sulfurized product containing in the range of about 5-25 weight percent sulfur is reacted with a mixture comprising about 30-90 parts by weight of lard oil and 70-10 parts of the polyisobutylene. Minor amounts (e.g. up to about 15 weight percent) of other unsaturated hydrocarbons (e.g. "C.sub.18 olefin") can also be present in the mixture.