I could use a little advice. I am working on a 65 Rivera with a posi rear end and I would like service it and don’t want to cause an issue with using the wrong oil. Factory manual states to use Buick 723 equivalent oil. That design really cross reference anymore. I have tried to do some research some say to use synthetic, some synthetic with the additive, others recommend non synthetic with additive. . Just don’t want to cause an issue using the wrong fluid. Thanks for the help.