It is generally recommended to use the lubricants specified by the manufacturer for your motorcycle. In the case of the Suzuki Gixxer SF 155, it is best to consult the owner's manual or contact a Suzuki authorized service center to determine the recommended oil viscosity and specifications for your specific model.
That being said, the Suzuki Gixxer SF 155 typically requires a four-stroke engine oil with the viscosity grade of 10W-40. While the Servo 20W-40 oil you mentioned may be within the acceptable viscosity range, it is important to consider other factors such as the oil's composition, additives, and specifications that meet the manufacturer's requirements.
To ensure optimal performance and protection for your motorcycle's engine, it is strongly advised to adhere to the manufacturer's guidelines for oil selection and maintenance.