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: