The paper deals with the results obtained during experimental research of the heat transfer performances of cross-flow heat exchanger with highly viscous fuel oil flowing normal to the tube bundle with Re < 1 and Pr similar to 2000. Experimental research was performed on the oil cooler that consist of 50 in-line, with 10 mm outside tube diameter, 100 mm tube length, with both longitudinal and transversal tube pitch of 19.5 mm. Comparison of experimental results with equations from literature sources showed that dispersion of measured and calculated values is significant. In order to obtain more accurate equation statistical analysis was performed and dimensionless equation in the form Nu(D) = 3.17 center dot Re-Dc(0.1) center dot Pr-1/3 center dot ((Pr)/(Prw))(0.25) is obtained. Similar form of equation can be obtained when Reynolds-Colburn analogy is applied on previous experimental research of pressure drop. (c) 2006 Elsevier Ltd. All rights reserved.