How much oil to add 81 Yamaha XS 400?

On the right-hand side of the engine is the oil inlet. A small dipstick should be attached to the screw-on top.
If the dipstick is missing, the XS 400 has a small see-though reticle on the side of the engine. When sitting on the center stand the oil should come up to at least halfway up the reticle, but can go as high as the top of the reticle. Be careful not to overfill the crankcase as excess oil will be vented via a breather value near your air filters.