g.m.-limited slip,diff...book calls for 75/90 syn...G.M. dealer ship wants $29.99 a qt., times 2.5qt. ... what about 4oz. of friction mod...doe's any one know what it is ? {zddp/moly,ect...} is mobil 1 75/90 syn. ok? $7.00 a qt. does it need friction mod.? thank's...