Methods are provided for determining the compatibility of various grades of fuel oils, as well as methods for modifying fuel oils to improve compatibility and improved compatibility compositions. It has been discovered that the toluene equivalent solvation power of a blend of fuel oils does not vary in a straightforward manner with respect to the toluene equivalent solvation power of the individual blend components. Instead, it has been determined that the asphaltene content of the individual components can also influence the toluene equivalent solvation power of the final blend. Based on this discovery, methods are provided that can allow for modification of one or more components of a potential fuel oil blend. This can reduce and/or minimize the likelihood of asphaltene precipitation when a fuel oil blend is formed.