Anyone here replaced a Rear Main Seal? Mine is leaking. It's not a cheap job for a shop, so the question is to either repair it or use a stop-leak Oil Additive? If I choose the later, what are the likely downsides or side-effects of using such a thing? Which one is recommended, if any?

I'm looking at the alternatives, since this is an older '95 3.4L

TIA!

 

it doesn't work...and it will most likely gum up your motor

 

As a general rule they all work the same way. They swell and soften the seal. If your car has been sitting and the seal are hard it may help but is a seal has started to tear this can be enough to make the seal completely fail.
your best bet is to watch it closly so you do not run low of oil and let it leak until you fix it or sell it.

 

What happens is the rubber frictions a groove in the crank end. You can catch your fingernail on the groove, some are that deep. There is a trick sleeve you lay over the old crank to seal area. You then press a new seal over the sleeve and theory goes this is now a flat surface so the lip has some grip.

The other trick is to (looks around so no one hears this), but you shave the outer steel of the new seal. Once you set the seal in place, the material you took off the backing of the outside shell, now walks farther into the engine block. The lip moves past the old groove (say nothing!).