Excessive oil consumption

You need to top up the oil between oil changes, or the level drops abnormally fast. One essential point first: on a two-stroke engine, consuming oil is NORMAL (the mix oil burns along with the fuel by design). This symptom therefore only applies to four-stroke engines, where oil and fuel are completely separate and oil consumption is abnormal. Two broad explanations: either oil is LEAKING to the outside (crankcase gaskets, drain plug, seals, hoses — you'll see marks under the engine), or oil is BURNING in the combustion chamber (it disappears with no trace on the ground, but produces blue smoke). For burning oil: worn rings (blue smoke on acceleration, under load), worn valve stem seals (blue smoke on startup that clears up once warm), or a failed head gasket (often with a milky residue, white smoke, overheating). You can tell them apart with simple observations: are there oil marks on the ground (external leak)? blue smoke, and when (rings vs. valve seals)? a milky residue under the oil cap (head gasket)? Urgency: keep a close eye on the oil level and top it up — riding with a level that's too low destroys the engine. Moderate consumption is worth monitoring, but significant consumption (more than roughly 0.5 L/1000 km) or consumption that's increasing signals internal wear that needs diagnosing, especially since it can foul the catalytic converter. Track your consumption to get an objective read on the problem.