Forty-one short-trip service tests were run using 1964-1969 model U.S. passenger cars and 19 commercial or experimental oils evaluating several aspects of engine rusting. These factors included differences in rusting severity among engine models, effects of fuel variables on rusting, and the effects of oil additive type and concentration on rusting.Rusting severity varied widely among engine models, but different engines ranked similarly the rust protection provided by oils with differing additive treatments.Use of either nonleaded gasoline or LPG instead of leaded gasoline greatly reduced engine rusting.At equivalent additive treatment levels, several magnesium sulfonates provided better rust protection than did certain calcium sulfonates, calcium phenates, or a barium sulfonate. Rust protection increased greatly and approximately linearly with increasing magnesium content.From these tests, and those reported by others, there appear to be three requirements for engine rusting to occur: 1. Addition to the crankcase of corrosive products from the gasoline lead antiknock scavengers and combustion products. 2. Cold engine operation, permitting condensation and accumulation of corrosive products in the oil. 3. Depletion of rust-preventing oil additives. Since the first two requirements are at present largely uncontrolled from the customers' standpoint (most gasolines are leaded and contain scavengers, and winter weather is cold in many areas), the only other apparent avenue for improving the rust protection provided to current-design engines using leaded gasolines is better rust-preventing additives in the oil.