You're correct that oil is not conductive, and under normal circumstances, it should not cause a short circuit between the leads of a motorcycle stator. However, there could be other factors at play that cause a short circuit specifically when the engine is filled with oil. Here are a few possibilities to consider:
Contaminated oil: If the engine oil is contaminated with conductive substances such as dirt, water, or metal particles, it could create a conductive path between the stator leads and ground. These contaminants can bridge the gap between the stator windings or between the stator and the engine case, causing a short circuit.
Faulty insulation: The stator windings are typically insulated to prevent contact with other components or the engine. However, if the insulation is damaged or degraded, it can lose its insulating properties. When oil is present, it can penetrate the damaged insulation, allowing a conductive path to form between the stator leads and ground.
Thermal expansion: Oil can expand when heated, and this expansion can put additional pressure on the stator or its wiring. If the stator leads or their insulation are already weakened or compromised, the expansion of the oil can force the leads into contact with the engine case or other conductive components, resulting in a short circuit.
Vibration and movement: Motorcycles experience significant vibrations and movement during operation, which can potentially cause wires or connections to shift or come into contact with each other. If the stator leads are not securely fastened or are positioned too close to conductive components, the movement caused by the engine's vibrations could lead to intermittent or permanent short circuits when the engine is filled with oil.
In any case, if you suspect a short circuit in the stator leads, it's essential to have the issue diagnosed and repaired by a qualified motorcycle technician. They can thoroughly inspect the stator, its wiring, and other related components to identify the cause of the short circuit and take the appropriate corrective measures.