Dexron III.
Also, don't cheap out on your coolant: putting the green stuff in will allow accelerated corrosion, and you will be writing here soon asking for help on chaging your leaking water pump.

Not another spark plug post
 

but there are lots of rants on the use of Bosch platinum plugs in older MBs. Most of the problems are with plats used in MBs that use waste spark ignition systems like M104 engines with +2 and +4 plats being the worst. At the risk of risking all out war, my experience with my 1991 300TE (M103) is that single and +4 plats performed at least as well as the stock plugs and lasted substantially longer. I also rountinely used Bosch single plats in my 126 Euro 380SE and Euro 500SEL with no problems and 25K plug life. Just my own experiences here, not extensive research. In short, IMO if your M103 runs well on the platinum plugs that you have installed leave them in until proven otherwise. Mark

Thanks, I got the Dextron III. I haven't approached the coolant yet, but I will remember to use a factory spec product. Its not leaking yet but I will take your word for it. :)

Hooked, yeah I am still curious, but I am not excited about pulling them back out unless I can get some clear direction on what harm they will do,

Been there done that. They are a marketing gimmick. Use the factory recommended parts, fluids and then have fun.

Well, they were only $2 each and they work. Can some say with some certainty that these plugs are going to cause some damage?

Do what you wish. It's your car. I had bad luck with them. They ran good for about 1500 miles and then started to miss. I took them out and all were fouled. Never again...

Just wanted to update that my local mechanic concurred with you guys and said the platinums would foul prematurely and that he felt the car would run better on the coppers so they are coming out.

Mechanic also said the water pump has already been replaced along with coolant hoses. I am guessing the thermostat was swapped out as well.