In order to obtain the data of the stability of high viscosity standard oils, the variation of viscosity with age was measured by means of a caoillary master viscometer for nolybutenes having viscosities of 55, 340, and 1010 poises at 25°C, and for polybutene-mineral oil mixtures having viscosities of 20, 100, and 540 poises at 25°C.The results obtained can be summarized as follows:(1) The viscosity increments for the test samples, which were stored under the condition that the volume of the samples in each 640ml bottle is 500ml, are less than 0.2%/year, except for the hydrogenated polybutene.(2) The viscosity increments for the test samples, which were stored under the condition that the volume of the samples in each 640ml bottle is less than 500ml, are larger than 0.2%/year, and the viscosity increments for a year are inversely proportional to the volume of the sample in 640ml bottle, except for the polybutene-mineral oil mixtures having viscosity of 20 poises at 25°C.(3) A correlation between the acid value and viscosity increment for a year exists for all the test samples.