It is generally not recommended to mix different types or viscosities of engine oils, as they are formulated with specific properties and additives that may not be compatible with each other. Mixing mineral oil (15W50) with fully synthetic oil (5W30) can lead to an unpredictable oil blend that may not provide the necessary lubrication and protection for your engine.
In the case of your Royal Enfield Thunderbird 350X, it is advisable to stick to the manufacturer's recommended oil viscosity and type, which is typically specified in the owner's manual. Using the correct grade and type of oil ensures optimal performance and longevity of your engine.
If you are uncertain about the appropriate oil to use or have specific concerns about your motorcycle's oil requirements, it's best to consult with a qualified mechanic or contact the Royal Enfield service center for accurate guidance pertaining to your particular model.