I’ve been asked several times whether or not it’s okay to use the standard auto oil in your outboard engine; the answer is a bit hazy. It is possible for the outboard engine to run off standard automobile oil, however, you wouldn’t want to do it for long. The problem is regular oil doesn’t contain the rust inhibitors which prevent marine engines from accumulating rust. In addition, it causes a lot more wear on an engine to operate in a marine environment versus on a relatively flat surface like a road, and normal oils aren’t formulated to withstand that kind of punishment.
The National Marine Manufacturers Association has introduced multiple tests to probe the quality of oil for marine engines, and as it turns out, only marine engine oil is strong enough to withstand the stress. Thus, if you’re in a situation where you absolutely need to use your boat and don’t have marine engine oil on hand… it’s okay, but afterward it’s best to flush your engine and replace the oil. There’s still risk involved regardless.