I have never found any additive that works worth spit. Many do more harm than good..esp. the ones solvent based. Are you sure its the main leaking? If it is, bite the bullet and replace it. While it may be 'small and messy' sitting still, remember that the main is a pressure seal, so under power the leak is larger and more problematic. Also, a leaking rear main can blow entirely at the worst possible moment....how much do you like your motor? First though, make sure that the
oil pan gasket is seated correctly, as a leak @ the back of the pan can be confused for a rear main. With that much compression, also be sure your pcv system is working corrrectly and you are getting proper venting. The only way I have ever found to slow a main leak is to use thicker
oil, which in turn strains the motor. Whatever you find, good luck!