A additive for high temperature naphthenic acid corrosion inhibition comprising a chemical mixture of: a) corrosion inhibiting amount of polyisobutylene phosphorous sulphur compound A; and b) corrosion inhibiting amount of thiophosphorous sulphur compound C, wherein said polyisobutylene phosphorous sulphur compound A is obtained by reacting an olefin with phosphorous pentasulphide in presence of catalytic amount of sulphur, forming a reaction mixture, wherein molar ratio of said olefin to said phosphorous pentasulphide is between 1:0.05 to 1:2, said olefin is high reactive polyisobutylene (HRPIB), said sulphur is sulphur powder, and wherein said sulphur powder is present in catalytic quantity of 0.5% to 5% of said olefin by weight; wherein said thiophosphorous sulphur compound C is of the formula 2, is obtained by reacting a thiophosphorous sulphur compound B with ethylene oxide, and wherein the formula 2 comprises resulting compound C obtained after reaction of said compound B with said ethylene oxide, wherein said thiophosphorous sulphur compound C includes mono-ester, di-ester, mixture thereof; wherein X is independently either sulphur or oxygen and at least one X is sulphur, and wherein R 1 and R 2 are hydrogen or hydrocarbyl having 5 to 18 carbon atoms; wherein said compound B is a phosphorous thioacid ester of the formula 1 wherein X is independently either sulphur or oxygen and at least one X is sulphur and wherein R 1 and R 2 are hydrogen or hydrocarbyl having 5 to 18 carbon atoms, wherein said phosphorous thioacid ester of the formula 1 includes mono-ester, di-ester, mixture thereof; wherein weight ratio of the compound A to the compound C varies from 1:1 to 4:1. a) corrosion inhibiting amount of polyisobutylene phosphorous sulphur compound A; and b) corrosion inhibiting amount of thiophosphorous sulphur compound C, wherein said polyisobutylene phosphorous sulphur compound A is obtained by reacting an olefin with phosphorous pentasulphide in presence of catalytic amount of sulphur, forming a reaction mixture, wherein molar ratio of said olefin to said phosphorous pentasulphide is between 1:0.05 to 1:2, said olefin is high reactive polyisobutylene (HRPIB), said sulphur is sulphur powder, and wherein said sulphur powder is present in catalytic quantity of 0.5% to 5% of said olefin by weight; wherein said thiophosphorous sulphur compound C is of the formula 2, is obtained by reacting a thiophosphorous sulphur compound B with ethylene oxide, and wherein the formula 2 comprises resulting compound C obtained after reaction of said compound B with said ethylene oxide, wherein said thiophosphorous sulphur compound C includes mono-ester, di-ester, mixture thereof; wherein X is independently either sulphur or oxygen and at least one X is sulphur, and wherein R 1 and R 2 are hydrogen or hydrocarbyl having 5 to 18 carbon atoms; wherein said compound B is a phosphorous thioacid ester of the formula 1 wherein X is independently either sulphur or oxygen and at least one X is sulphur and wherein R 1 and R 2 are hydrogen or hydrocarbyl having 5 to 18 carbon atoms, wherein said phosphorous thioacid ester of the formula 1 includes mono-ester, di-ester, mixture thereof; wherein weight ratio of the compound A to the compound C varies from 1:1 to 4:1. wherein X is independently either sulphur or oxygen and at least one X is sulphur, and wherein R 1 and R 2 are hydrogen or hydrocarbyl having 5 to 18 carbon atoms; wherein said compound B is a phosphorous thioacid ester of the formula 1 wherein X is independently either sulphur or oxygen and at least one X is sulphur and wherein R 1 and R 2 are hydrogen or hydrocarbyl having 5 to 18 carbon atoms, wherein said phosphorous thioacid ester of the formula 1 includes mono-ester, di-ester, mixture thereof; wherein X is independently either sulphur or oxygen and at least one X is sulphur and wherein R 1 and R 2 are hydrogen or hydrocarbyl having 5 to 18 carbon atoms, wherein said phosphorous thioacid ester of the formula 1 includes mono-ester, di-ester, mixture thereof;