Abstract: One aspect of the present invention is configured in such a manner that a main valve (a valve member) is in abutment with only a ridge portion (a second protrusion portion) on the inner peripheral side of a seat portion (a first seat portion) in a state of being mounted on a piston. This configuration can reduce the radial contact width between the main valve and the seat portion, and contribute to preventing or reducing variations in the valve-opening point and the damping force by managing the radial dimension of the ridge portion on the inner peripheral side of the seat portion to allow the contact diameter between the main valve and the seat portion to be kept constant.

Abstract: A base oil for lubricating oil, a lubricating oil composition containing the base oil and a phenol-based antioxidant and/or organomolybdenum compound, and also an additive for a base oil for lubricating oil, consisting of a phenol-based antioxidant and an organomolybdenum compound are disclosed. The base oil and the lubricating oil composition containing the base oil are stable against NO.sub.x gas and are useful for use in internal combustion engines. The additive, when added to a base oil, provides a lubricating oil which is stable against NO.sub.x gas and can be used effectively in a NO.sub.x gas atmosphere.