The effects of API SE engine oils on the activity of a 1975 prototype monolithic, noble metal oxidation catalyst have been studied in long-term multicylinder laboratory engine tests. Results of this work showed that oil consumption rates, up to 1 qt/1000 miles, and lube ash up to 1.6%, do not significantly affect catalyst activity in 500 hour tests, equivalent to 20,000 miles (32,180 km).A commercial type 1% ash SE oil showed only a small effect on catalyst activity in an extended 1250 hour test, equivalent to 50,000 miles (80,450 km). At the end of the test, the catalyst met the original 1975 emissions standards of 3.4 g/mile CO and 0.41 g/mile HC.A higher-than-normal phosphorus-containing oil (0.32% P) caused significant loss in catalyst activity after testing for 30,000 miles (48,270 km). This suggests that any need for increased phosphorus-containing additives in future oils could cause catalyst poisoning before 50,000 miles.This work provides added assurance for the compatibility of SE oils with oxidation catalysts on 1975 cars. However, reactor-equipped vehicles in actual service undoubtedly will provide a more severe test of the lube effects on catalyst life and durability.