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To answer one of the original questions in this post, I do not believe that the 3.5/3.4L engines in any BMW 5-, 6-, or 7-Series require premium fuel. According to BMW, regular can be used. However, the smaller 6-cylinders (the 2.5L and 2.7L like in the 528e I had) require at least 91 octane. I assume this has to do with a higher vs. lower compression ratio. So a 1992 535i should be fine with regular, even by BMW.
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