The exact amount of engine oil required for a Royal Enfield Thunderbird can vary depending on the specific model and engine displacement. However, as a general guideline, most Royal Enfield Thunderbird motorcycles require around 2.5 to 3.0 liters (or approximately 2.6 to 3.2 quarts) of engine oil for a complete oil change.
To get the most accurate and up-to-date information for your particular Royal Enfield Thunderbird model, it's recommended to consult the owner's manual or contact a Royal Enfield dealership or authorized service center. They will have the precise specifications for the engine oil capacity and type required for your specific model and can provide the most accurate guidance.