Does anyone have any experience in cleaning the reservoir to get rid of both the stains and the gunk?  I have heard that some degreasers (Purple Power, Orange, Simple Green) may work with bleach and coarse sand or kitty litter after vigorously shaking the solution in the reservoir.  It would help if any one has experience or suggestions with these or other concoctions to get rid of this mess.

Kettsvette said: My '88 has a thin film of gunk in the upper radiator area of the cooling system and the reservoir.  I believe this gunk is a result of the local Chevy dealer using DEX-COOL instead of the ethylene based "green" conventional antifreeze about 2 years ago that went unnoticed by me.  The reservoir has become internally stained and the greasy gunky film is also inside the reservoir.

Bleach water + rag on coat hangar or pea gravel and shake.

 

then a soap/cleaner solution + rag on coat hangar or pea gravel and shake if needed.

I did this on my RS because of rust staining.

 

After I removed the rocks and cleaning solution and rinsed the reservoir several times, I used the bleach to whiten the plastic and placed the reservoir in the sun to dry and get the benefits of the sun to help with the whitening.  After about 5 hours of drying I again rinsed the reservoir and it looks pretty close to new.

 

Again, thanks for the reply and your point about mixing a bleach with solvents is a valid precaution.