The Yamaha TTR 125 requires motorcycle-specific oil with a JASO MA or MA2 rating. This ensures that the oil has the correct additives to protect the transmission and clutch of the motorcycle. Car engine oil, such as 10w40, does not have these additives and can cause damage to the transmission and clutch.
In addition, 10w40 oil is designed for use in cars, which operate at higher temperatures than motorcycles. The lower viscosity of 10w40 oil can cause it to break down more quickly in a motorcycle engine, leading to engine damage.