On the 318 I'd use 98 if you do much hard driving. The e30 M42 does not have the knock sensors that the e36 versions had, and lower octane fuel will ping/nock/detonate in our version of the motor at high rpm or large throttle inputs. The 100 octane fuel probably wouldn't hurt, but I try to avoid using anything with ethanol in it in an older car. Ethanol is very hard on all of the rubber parts of an engine/fuel system, especially if it was not designed for ethanol use. Also, ethanol doesn't have the stored power that gasoline has, so it doesn't produce as big of a bang.