If it doesn't "ping" severely no problem
From your question I assume you have the supercharged 2.3 liter. One tank of 87 octance will not hurt it,but DON'T do it on a regular basis. The higher combustion chambers temps that result from knock(this happens because the incorrect fuel is being used) can destroy pistons and even burn holes in them. Usually this isn't a problem because as soon as the knock sensor detects engine knock it tells the ECU to alter ignition and timing maps to correct the problem,this only delays knock from happening, in the end it is better to use the correct fuel which is 91 octane and above.
Premium only
It is recommended and you might get better performance and more life out of it. But you can run regular fuel in it
The S denotes the 2.3 Supercharged engine one the Mazda millenia (rather than the regular 2.5)
Mazda Millenia hoods from models 1995 to 2002 will fit on a 1995 Mazda Millenia. The Mazda Millenia was first launched in 1995.
Mazda Discontinued the Millenia in 2002.
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The 626 and the Millenia share the same 2.5 liter engine,the only difference being the version in the millenia makes more power.