More about why you're asking further down, but to initially address your question, there is really nothing that you can use.
(Yes, a Red-Line brand product called Water Wetter is an additive that breaks down and prevents the formation of bubbles on the walls of the cooling passages inside your engine -- like bubbles on the sides of a pot of water that's about to start boiling -- because the gas bubbles block the efficient transfer of heat to the coolant, thus slightly enhancing cooling efficiency).

The reason is that water has the highest latent and specific heat of any common liquid that you could use in your engine, meaning that it can absorb a LOT of heat, and will do so without getting very hot itself (and boiling, which you don't want because that severely reduces the transfer of heat to the radiator); and thus it's very good for carrying heat from the engine to the radiator. However, water alone is prone to boiling (and freezing), and thus needs additive (which we call antifreeze), even though antifreeze actually reduces the amount of heat that the water can transfer to the radiator. It's a tradeoff.

I have heard of an organic chemical that has been used in car cooling systems that isn't prone to boiling over, but it's expensive and has some drawbacks.

But let's address your problem, which you no doubt believe you have because you're asking this question.

As you've probably read in other inquiries on this forum, the first thing is to determine whether you truly have a cooling problem, because 1985 to 1993 240s have an annoying little circuit board attached to their temperature gauge called a Temperature Compensation Board, and when it gets old it lies! It makes a lot of folks think that they have a problem -- in fact, I bought one of my '93's from a guy who was desperate to sell his car because it would keep "overheating". When I checked it out, I brought a little IR thermometer and, scanning the thermostat housing, discovered that the car's temperature was perfectly normal; and then I bought it for a very cheap price.
Then, when I got the car home, I removed that TCB, just like I've done with my other '85-on cars, so that the gauge shows true temperature like it should.

However, if you do have an overheating problem, there are many possible reasons: bad thermostat, clogged radiator or engine cooling passages, failing water pump, failing fan clutch, etc. Each has its own distinct "signature" or set of symptoms and associated conditions under which the overheating occurs. Tell us more about your problems and we'll try to help.

There isn't anything out there that will cure a stuck thermostat, clogged radiator, or water pump that isn't performing like it should. That being said, there is a product from Red Line that helps reduce surface tension on water and helps the radiator do its job. Available at most large auto stores.