In methods of preparing asphalt and elastomeric polymer compositions crosslinked with phenol aldehyde resins and sulfur, such as polymer modified asphalt (PMA), it has been discovered that the MP1 compatibility may be improved by adding certain ionic metal oxides. Suitable ionic metal salts include, but are not necessarily limited to, zinc oxide, cadmium oxide, and the like. Acceptable elastomeric polymers include, but are not necessarily limited to, styrene-butadiene copolymers. Additional sulfur-containing crosslinkers may also be used.