The daylight stability, foaming characteristics, color, engine performance test behavior, oxygenates content, and thermal stability of wax isomerate oils and/or hydrocarbon synthesis liquid products are improved by the process of contacting the aforesaid wax isomerate oil and/or hydrocarbon synthesis liquid products with a silica adsorbent, said silica adsorbent being characterized by possessing a pore size of at least about 100 .ANG., preferably at least about 125 .ANG., most preferably at least about 150 .ANG., an alkali/alkaline earth ion concentration, excluding sodium, of greater than about 125 ppm, an iron content of less than about 40 ppm and a zirconium content of less than about 130 ppm.