What oil goes in a 1981 Honda 400?

Use an SE or SF type oil within the range of 5W-30 to 20W-50. (The lower the number, the better the engine will start in cold climates.) A 10W-40 has a constant viscosity and will perform efficiently across a wide range of engine operating conditions. Higher numbers are usually recommended for engine running in hot weather conditions. For more complete information, download the manual. You can download the entire repair and service manual from hondatech.info in Adobe Acrobat PDF format at: http://downloads.hondatech.info/Moto/CB/CB400/Honda%20Cb400%20Cb450%20Cm400%20Cm450%20Twins%20Nighthawk%20%281978-1987%29%20-%20Service%20Manual%20400%20Pag%28Eng%29.pdf