The rate of fouling and coking, in refinery crude and preheat trains has increased as the purchase of opportunity crudes has become more common. This has been discovered to be caused by the precipitation of asphaltenes on blending of incompatible crudes. Typically, one of the crudes contains insoluble asphaltenes while another crude is paraffinic. Consequently, laboratory tests for measuring the degree of asphaltene insolubility and the solvency of each oil has been developed. A solubility parameter based model provides a conservative estimate of oil compatibility. As a result, many examples of incompatible crudes have been identified and the order of mixing is found to be quite important. In addition, the blending of incompatible processed oils can coke heat exchangers and flaking of this coke can plug downstream fixed bed reactors.