This work studied the deterioration of commercial Frytol oil in controlled frying conditions using the anisidine index (AI) as standard evaluation method. The effect of temperature and time was studied at 170°C for up to 10 hours and at 200°C for up to 6 hours in the presence of factors which speeded up oil deterioration: 1% and 8% water based on the mass of oil and non-lipid material, 1% and 6% based on the mass of oil. Air flow rate was kept constant at 25 L h-1. The relationship between AI, colour and viscosity was studied to evaluate the technical viability of measuring these variables as a function of time to be used as a method for rating oil deterioration by small and medium sized enterprises (SME). Oil deterioration was higher and faster at maximum study interval values for each experimental factor, although only the effect of temperature was significant (p<0.05). Red and yellow on the Lovibond scale were weakly related (AI R2=0.558 and R2=0.526, respectively), colour depending on frying temperature. Viscosity measured at 30°C had the highest coefficient of determination (R2=0.809) and was independent of frying temperature. Viscosity is thus a variable which can be used for determining frying oil deterioration in SME as a consequence of a significant change from 10 cP to 45 cP which is closer to frying oil deterioration limit (AI 156). It can facilitate determination and lower associated costs, there-by improving product quality and customer confidence in them.