Engine Sludge has reappeared in the last decade as a source of operation problems and in manufacturers warranty claims in Europe and the USA, due to engine malfunction and in some cases engine failure through oil starvation. This sludge has become known as ‘Black sludge’ or ‘Hot sludge’ in Europe. As a result of the problem, bench engine tests have been developed in Europe (CEC-L-41-T-88), and the USA (ASTM Sequence VE). Both these tests have been shown to be particularly sensitive to changes in the fuel composition, even between batches of the same gasoline.A field trial has been conducted by Shell Research Ltd at Thornton Research Centre, to study the effect of fuel composition and fuel-lubricant interactions on the propensity to form sludge, using a mileage accumulation cycle designed to be severe with respect to sludge formation.Fuel compositions were taken at the extremes of levels currently found commercially, and the effect of a gasoline detergent additive was also included in the study. Oils with performance meeting API SF and API SG levels were used. The test was designed using a statistical half replicate approach with sixteen vehicles. Four additional vehicles were included to provide a reference to the Sequence VE test.The results show clear fuel and fuel-lubricant interaction effects on sludge formation and piston cleanliness. The controlling fuel parameters were found to be different for sludge formation in the rocker cover and sump areas. The use of a fuel detergent additive has been seen to have a beneficial effect on sludge control in these vehicles.