I think you should be ok with 87. Some people (like me) think that on older engines with higher miles, its worth running silghtly higher octane. Why? because the carbon deposits in your cylinder can actually take up enough room to make a difference on compression within your cylinder. Higher compression + low octane gas = pinging = knock = detonation = boom.
honestly, i'd say you're fine with 87. i run 91 in my stock 328. uneccessary? mabye. safe? forsure.
(dont run too high gas though, if anything i'd say run 89)
then again, what do i know
wait for someone else to confirm, i dont know anything about e21's specifically
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