Methods and systems for hydroprocessing heavy oil feedstocks to form an upgraded material involve the use of a colloidal or molecular catalyst dispersed within a heavy oil feedstock, a hydrocracking reactor, and a hot separator. The colloidal or molecular catalyst promotes hydrocracking and other hydroprocessing reactions within the hydrocracking reactor. The catalyst is preferentially associated with asphaltenes within the heavy oil feedstock, which promotes upgrading reactions involving the asphaltenes rather than formation of coke precursors and sediment. The colloidal or molecular catalyst overcomes problems associated with porous supported catalysts in upgrading heavy oil feedstocks, particularly the inability of such catalysts to effectively process asphaltene molecules. The result is one or more of reduced equipment fouling, increased conversion level, and more efficient use of the supported catalyst if used in combination with the colloidal or molecular catalyst.