Synthetic lube and leather seals

I've never seen a leather pinion seal on an 8-3/4 and I've tore apart ones dated as old as 1966 that I'm sure were original. Who's telling you their made of leather?

Was the yoke checked good for wear? I've seen a few with a groove worn in them right where the seal rides and they won't seal up. Also saw them leak around the outer perimeter of the seal so I use a little shelac there when I install them. Haven't ran accross it but I've read that they can also leak out through the splines so guys use a little sealer on the splines.