There is no way to know if you are running dot 3 or 4 by just looking at it. I would assume it is dot 4 and top up with dot 4. If you are worried about the brake fluid you can change it out for known dot 4 before the heat of summer, so you can be sure of using the higher boiling fluid when the danger from overheated brakes is at it's max.

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'97 E 300 D