the cap you need is size B, available at autozone for a few dollars; walmart also sells one under the valvoline brand name. use it with a 3/8" drive ratchet.

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87 300e (white/black; amg body kit)
88 300ce (red/cream; amg body kit)
93 300ce cabrio (white/blue/blue top)
93 300ce cabrio (black/grey/black top)
98 ml 320 (totaled @ 137,000 miles)
99 clk320 (black/grey/black top)