A rear crankshaft seal (also called a rear main seal) that is wearing out can be hard to detect, depending on how much wear it has. Most commonly, it will show up as a small oil drip on the driveway or garage floor. You may also find engine oil collecting on the underside of the transmission or around the rear of the oil pan. In order to prevent total failure, it's a good idea to closely inspect the seal when either the engine or transmission is removed, such as during a clutch replacement, and always replace it at the first sign of wear.
Datsun 240Z, 260Z, 280Z, 280ZX
Datsun S30, S130
1970, 1971, 1972, 1973, 1974, 1975, 1976, 1977, 1978, 1979, 1980, 1981, 1982, 1983