To provide a system capable of suppressing wear due to friction in a friction system between two objects to the minimum, suppressing wear due to friction, in particular, between a cylinder and a piston of an internal combustion engine to the minimum, and extending overhaul interval to the maximum.

This system is provided with two objects performing contact and periodic relative movement, lubricating oil 3 provided on a contact face or a friction face 2 between these two objects, and insoluble content 4 that the lubricating oil 3 contains. A percentage content control means for adjusting and holding that percentage content of the insoluble content 4 for the lubricating oil 3 becomes a specific value is used. The lubricating oil 3 is used in such a way that percentage content p is expressed as function of the maximum revolutionary number n, that is, p(n) and percentage content P of the insoluble content 4 for the lubricating oil including a scope of effective error becomes a value given by P(n) = p(n) ±0.05 mass %.