I was told to be sure its "USED diesel engine oil". Cannot speak to the science however it was explained to me that the chemistry changes as diesel is used inside an engine. That chemistry change enables it to eat at rust and lubricate somehow. All I can attest to is that it worked twice for me. Both in 1947 engines that sat for well over 30 and 10 years. Never dismantled the engine at all other than the fuel system and cleaned the carb.