I would bet that your 81 SD does not have a converter. My '82 240D doesn't.
Yes, the trap oxidizer on the 300SDL was removed. However, in removal process, MB installed a catalytic converter down further down the exhaust pipe that is located next to the transmission. I looked underneath the car and counted three objects along the exhaust pipe (the catalytic converter, the mini pre muffler, and then the muffler towards the back). If you look underneath at your '87 300, you will see the converter located adjacent to the tranmission, unless your setup is different that mine. But if you have been using additives all this time and have no ill effects then maybe the Redline additives don't have those metals.
Loubapache, could you tell me what type of additive you've been using, how long you've been using it and (if it isn't too much trouble) check to see if you have a catalytic converter? I would like to use an additive but I'm afraid that I would be damaging the converter.
Also, does anybody else on the forum that has this same six cylinder engine (603) with cat. converter have any ill effects from using additives, please chime in.