Provided is a process for preparing base oil from a waxy hydrocarbon feedstock by contacting the hydrocarbon feedstock in a hydroisomerization zone under hydroisomerization conditions. The reaction is in the presence of hydrogen and an inert gas, with the total pressure in the hydroisomerization zone being at least 400 psig. A product from the hydroisomerization zone is collected and separated into base oil products and fuel products. The inert gas can comprise any suitable inert gas, but is generally nitrogen, methane or argon. Nitrogen is used in one embodiment.