The rear main oil seal or crankshaft seal is located on the gearbox end of the crankshaft. This seal can leak and oil can come in contact with the clutch friction plate can degrade its material in the manual transmission vehicles.It can potentially cause significant internal engine or turbo damage if the oil level of the engine becomes low. By replacing the seal, you can prevent oil leakage and preserve your engine's health. It's advisable to replace this seal during a clutch replacement job, even if it hasn't failed yet, to avoid having to remove the transmission again to replace.