Cat 235c / 3306 Bad Oil Leak

Hi, my father-in-law has a 1990 Cat 235c with a 3306. SN: 5AF00998.
I help him when I can, but we don't have a great amount of knowledge about these things, and don't have a manual for this excavator.

He tells me that this machine has an oil leak (which he estimates at about 18 quarts an hour). It's leaking oil at the point between the engine and the transmission. The best I can figure is the rear crankshaft seal needs replacing. Given the quantity of oil leaking, is that a reasonable...

Cat 235c / 3306 Bad Oil Leak

Cat 235c / 3306 Bad Oil Leak