Using engine oil with a different viscosity than what is recommended by the manufacturer can affect the performance and longevity of your motorcycle's engine. While I don't have the specific information regarding Royal Enfield's recommendations for the Classic 350 model, it's generally advisable to follow the manufacturer's guidelines for engine oil viscosity.
The viscosity rating of engine oil is designed to provide the appropriate lubrication and protection for the engine under various temperature conditions. The numbers in the viscosity rating represent the oil's flow characteristics at different temperatures. The first number (15W in this case) indicates the oil's viscosity at colder temperatures, and the second number (40 or 50) represents its viscosity at operating temperatures.
Using a higher viscosity oil, such as 15W50 instead of 15W40, may result in a thicker oil film, which can potentially impact the engine's lubrication, particularly during colder starts. Conversely, using a lower viscosity oil may not provide adequate protection and lubrication at higher temperatures or under heavy load conditions.
To ensure the best performance and longevity of your Royal Enfield motorcycle, it is generally recommended to use the engine oil viscosity specified in the owner's manual or recommended by the manufacturer. If you're uncertain or have specific concerns, it's always best to consult with authorized Royal Enfield service centers or contact their customer support for accurate and up-to-date information regarding the appropriate engine oil for your particular model.