Transmission / gearbox oil leak

You see traces of oil around the gearbox, the output sprocket, or, on a shaft-drive bike, the rear final drive and drive shaft. This is rarer than an engine oil leak, but shouldn't be ignored: a gearbox or final drive losing its oil risks a costly seizure. Important note: on most motorcycles, the gearbox shares the engine's oil (common sump); a "gearbox" leak is then often actually an engine oil leak located on the transmission side. On others (and on shaft-drive bikes), the transmission has its own oil. Main causes: a worn output shaft seal on the gearbox (oil seeping onto the sprocket and chain — common on 4-strokes), a gearbox case/cover seal (on the shift side or clutch side) that's failing, a poorly tightened drain/level plug (a slow leak, easy to fix), and, on a shaft-drive bike, drive shaft seals leaking (final drive oil around the rear wheel — a specialized job). You tell them apart by locating the traces: around the sprocket/chain? at the case/cover junction? at the plug? toward the rear wheel (shaft drive)? And by watching whether the oil level drops. Urgency threshold: a slow leak without a noticeable level drop isn't urgent, but keep an eye on the oil level (gearbox or engine oil, depending on the case) and don't ride if it drops — a transmission running low on oil can seize. Clean the area to better pinpoint the active leak: telling a genuine leak (seeping from behind the sprocket cover or from a seal) apart from simple chain-grease spray is key to a correct diagnosis.