It's called a "rear main bearing seal" and they're a P I T A to change! It involves dropping the oil pan, loosening all the main bearings, removing the rear main bearing cap, and coaxing the rope seal out of the groove above the crankshaft. There's a corresponding piece of braided seal in the main cap. Check the tech tips on this site and you'll find a lot of information about resealing a rear main, and some disappointing facts about how unreliable most of the ones available today are since they took the asbestos out of the seal ropes.

One particular brand is better than the rest, but I've got CRS disease right now. Maybe somebody else can help out!
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!