Biostability of Synthetic Crankcase Oils

The biostability of used and fresh samples of Lenoil GTX 15W/40 synthetic motor oil and Lenoil GLX 20W/50 synthetic motor oil were examined to investigate potential biodeteriogens and the stability of synthetic crankcase oils to degradation by microbial isolates obtained from them. The degrading bacteria were isolated via enrichment culture technique. Flow rates were established by determining drop rates and the growth profiles were established by monitoring the degree of turbidity, pH, optical densities and total viable counts. It was determined that used oils dropped faster than fresh oils at both low and high temperatures. Isolates from both groups of oils belonged to the genera Bacillus, Actinomyces, Edwardsiella, Pseudomonas, Micrococcus and Citrobacter among the bacteria and Aspergillus, Cladosporium, Cephalosporium, Penicillium and Mucor amongst the fungi. The used oil proved to be a better substrate for microorganisms. Fungi had higher counts than their bacterial counterparts. Used samples had higher microbial counts than fresh samples. The mixed culture of Actinomyces, Edwardsiella, and Pseudomonas, as expected, utilized the oil samples more readily than single cultures. For the single cultures, Pseudomonas showed the greatest degrading capacity for the used crankcase oils while Edwardsiella excelled with regards to the fresh oil. Observed pH dropped from 7.2–7.0 to 6.88–6.0 on average across all the samples. Synthetic crankcase oils are susceptible to attack both prior to use and when in use; it is, therefore, advisable to change the oil regularly to preserve the motor engine.