A method for stabilizing asphaltenes in petroleum feedstocks such as crude oil includes adding to the feedstock an effective amount of an additive containing at least one waste plastic. Suitable waste plastics include, but are not necessarily limited to, polyethylene, polyethylene terephthalate, polystyrene, polycarbonate, polyamide, and polyurethane, and combinations thereof. By “stabilizing” is meant keeping the asphaltenes in solution in the petroleum feedstocks.