The best oil weight to use for a Massey Ferguson Regent 500 2500 series tractor is 15W-40. This type of oil meets the manufacturer’s performance standards and provides better protection for the components of the engine. Additionally, this oil weight will help the engine to last longer and provide better fuel efficiency. For best results, it is recommended to use an oil with the appropriate viscosity grade that meets or exceeds the tractor’s performance requirements. Always consult the owner’s manual before selecting an oil weight.