'85 300TD wagon.
While doing a full coolant flush I noticed that my (original) coolant reservoir tank is not only old looking, but beginning to develop cracks. It is not leaking at this time, but I am not expecting it to make it through the winter.
I pulled a very nice (practically new) MB reservoir out of a car in a junkyard. The only real issue with it is that there is oil residue coating the inside of the tank.
I have tried to clean it with the following products (diluted) by putting them in the tank, adding water, shaking, flushing, even letting it sit overnight full of water/cleaner:
-Totally Awesome
-Dawn dish soap
-vinegar

The amount of slime inside has diminished, but there is still a lot of residue left.
Can anyone recommend something that might clean it a little better, or completely? I do not want to damage the tank or any o-rings (obviously). I was thinking of trying Simple Green next.
I can't spend the ~$50 on a new tank and plug at this time. I also don't want to install this one and have it cause problems or ruin my new coolant.
Thanks for any input.
Jon