Bentonite grease it a thick, black, sticky, tar-like grease.
Basically, before they put the 2 halves of the transaxle case together, they pump the specified amount in. The only way to "change" it would be to take the transaxle out, split the 2 case halves apart, take all the shafts and gears out, and soak everything in degreaser, repack with grease and reassemble the unit. It is not really necessary or practical to do as part of routine maintenance. It really only is done then the transaxle is apart for repair. I wouldn't worry to much about it, my grandmother has a late 80's John Deere STX38 rider, I had the unit apart to replace the shift keys in the transaxle, it still had plenty of grease in it after all these years.
Transaxle reliability depends on who manufactured your Craftsman, some were made by AYP (American yard products), some were made by MTD. MTD made their own transaxle, they were marginal even under light duty use. AYP used Peerless transaxles, which have been pretty reliable units (thats what was in the STX38)