Abstract: An improved catalytic process for heavy hydrocarbon conversion, (usually, but not necessarily, in the presence of nickel and vanadium on the catalyst and in the feedstock.) to produce lighter molecular weight fractions. Manganese, which has paramagnetic properties, is added so it progressively accumulates on aged catalyst, and enhances magnetic separation of aged catalyst, to increase activity and improve selectivity of remaining catalyst which is recycled. Manganese acts as a "magnetic hook" to separate more magnetic, older, less catalytically active and less selective, higher-metals-containing catalyst particulates from less-magnetically-active, lower-metal-containing, more catalytically active and selective catalysts fractions, which are then recycled back to the unit.