Yes its either the slave cylinder leaking so keep an eye on your brake fluid level or its the Crank shaft oil seal, both jobs require the box to be removed, i would say its the crank shaft oil seal, which is a flywheel off jobbie. Mine was weeping on my ST170 so i changed it along with a new fly wheel, clutch and slave cylinder.
If your going to take the box off i would replace clutch and fly regardless of age

Second thoughts check your thermostate too, these can leak when the engine is running and the coolant leak may make its way on to the cat, but if its 100% oil its crank shaft seal