The novel synthetic polyethylene fluids of the present invention are characterized by high viscosity indices (about 94-151 VI), low pour points (about -60 to -10.degree. C.), and a kinematic viscosity of about 6 to 28 cSt KV at 100.degree. C. The product compositions comprise polymers having a molecular weight of from about 300 to about 30,000 and a branching index of 151 to 586 branches per 1000 CH.sub.2 groups, or BI=0.18-0.40. A process for preparing such novel synthetic polyethylene fluids comprises polymerizing ethylene in the presence of a catalyst system, said catalyst system comprising [A] and [B], wherein [A] is a catalyst having the formula MX.sub.5 and [B] is a co-catalyst having the formula R.sub.n AlX.sub.3-n where M is a Group VB, VIB, or VIII transition metal; X is a halogen anion; R is a C.sub.1 -C.sub.20 alkyl group; and n is 1-2.