I'd fill it with premium. The higher octane burns a little hotter... but the thermostat isn't going to let the car get much hotter. If there's a considerable amount not being burned, that will heat up the cat a little more.
But only a little.

The only difference between something that's 87 and 92 octane is that it is slightly more resistant to detonation without a spark because it acts like a ratio of 92 octane to x amount heptane instead of 87 octane to a higher amount of heptane. There is fuel with octane ratings of 115 and higher, so it's not a percentage.

The bottom line is that it won't matter much, but premium gas might be a little better.
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1990 740 Turbo, on its way to stock specs, maybe beyond