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For a diesel lubricant to meet the new Mack EO-K/2 specification, it must be effective in preventing excessive viscosity increase during the 150-hour Mack T-7 test. The severity of this test is shown to be highly dependent upon fuel chemistry and injection timing. A comparison of various lubricant formulations in the Mack T-7 engine run with a given fuel suggests that nitrogen-containing succinimide dispersants, dispersant viscosity improvers, and supplemental ash in the form of overbased sulfonate detergents are effective in controlling viscosity increase. Crankcase oil thickening follows a modified form of Brinkman’s equation and can be predicted from measured values of soot particle size and concentration. Basic lubricant additives are shown to prevent particle size growth by adsorption on to the acidic soot surface, thereby interrupting soot aggregation and retarding oil thickening.