The changes in oxidation stability of two grades of crankcase lube oil, monograde (SAE 40) and multigrade (SAE 20W50), and their base oils were monitored by measuring their carbonyl peak index with respect to time in order to corroborate the results of sludge deposit and soluble acidity characteristics. The carbonyl peak index was calculated as the difference in the ratios of the absorbance of the carbonyl band at 1700 cm(-1) and of the basic peak methyl stretching vibration at around 3000 cm(-1) at oxidation time t and the start of the experiment. The results showed that the base oils were oxidized to a greater extent than the formulated oils. The monograde oil and its base oil had higher carbonyl peak indexes than their respective multigrade counterparts, showing that the monograde oils were less resistant to oxidation than the multigrade oils. However, results of sludge deposits and soluble acidity showed that the multigrade oils formed more oxidation products than the monograde oils, which also incidentally suggests that the multigrade oils deteriorate faster. The two conflicting observations therefore suggest that the types of oxidation products formed from the two oils were different even though initiated by similar mechanisms. The effects of metals (zinc, nickel, and aluminum), as well as the effect of water (on the activity of the metals), on the overall carbonyl peak index are reported.