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Late answer to an old question. The differences between the OMC 1974-ca. 1992 9,9 and 15 are twofold. The 9,9 can be upgraded to 15 ps with a carburetor change and also by removing and enlarging the intake manifold opening behind the carb on the 9,9. Plus, on some models the exhaust tuner/megaphone differs. The 15 develops its hp/ps at 6250 RPM, and above 6000 RPM you should run Evinrude or Mercury oil at 25:1, not at 50:1 as originally recommended. I have written about these differences over the years in articles for 'The Antique Outboarder', the quarterly publication of the AOMCI. We own, among many OMC motors of different sizes, a 1981 Evinrude 15 that we ran at 6500 RPM for 6 seasons. The motor idles perfectly with a 25:1 mix using Evinrude oil. We ran fully synthetic XD100 oil because it lubricates the best and burns the cleanest, leaving no carbon deposits in the exhaust porrts, piston crowns, or exhaust tuner.
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