The rate of piston ring wear of treated B heavy oil (LS) in ultrasonics was smaller than untreated one like Fig. 2 and 3. The wear when using several kinds of lubricating oil together with treated or untreated B heavy oil (LS) is shown in Fig. 3, and the effect of dispersant content in lubricating oil on the rate of wear was remarkable. Especially, when treated heavy oil was used with dispersant containing lubricating oil, the rate of wear decreased considerably.