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OMC Cobra's must use straight 30W oils. You cannot use multi-viscosity oil. The additives that are put in multi-viscosity oils cause the oil to break down too fast. This will cause the oil to lose its lubricity characteristics. Remember, a stern drive engine application is much different than a car or truck - the stern drive engine normally sustains a higher running rpm for a longer period of time than a car or truck ( most cars or trucks cruise 2500 rpm's or less, where a stern drive will run 4000 rpm or more ). Using a synthetic oil is acceptable if a straight 30W cannot be found. The change interval remains every 50 hours! Therefore, given the higher cost of synthetics, this doesn't make them cost effective for use by most boaters.