This invention provides a process for upgrading a variety of hydrocarbon oils including low-grade crudes and fractions thereof. In this process, a hydrocarbon oil having an ASTM 50% temperature not higher than 550.degree. F. is converted at low temperature and pressure to more volatile products by a dispersion of crystalline zeolite catalysts having a silica: alumina ratio of at least 12 and a C.I. within 1-12. Initially, 0.02-10 wt % of the catalyst is dispersed in the feed until the catalyst inventory in the reactor stage accumulates. Thereafter, catalyst is added and removed to maintain a total catalyst content not greater than about 35 wt % of the feed in the reactor.