Coolant leak

First, Dexcool is still a glycol based coolant.  The difference between it
and the traditional green formulations is the additive package.

There are some antifreeze formulations based on propylene glycol.
It is still a glycol, but is less poisonous than ethylene glycol.  It is
an inferior coolant when compared with ethylene glycol, but it works
well enough.

I would, if it were mine, try to find out if there is really a leak.  I dont
like to add stop-leak compounds except in case of emergency.  They
can precipitate in the cooling system, but they probably wont ruin your
engine.   It is just a poor way to handle the problem if you have an
alternative.

In the really hot months of the year, you may lose a bit more coolant
than in the cooler months.   The hot coolant expands into the catch
container, and some small amount of evaporation is inevitable.

You, or a qualified mechanic, can often get under the car and look for
rusty streaks to see if there is really a leak.  It doesnt take much of a
leak to add up to a few ounces every few weeks or months.

There is always potential to cause harm with additives.  If it is going to
plug a leak, it may also plug cooling passages over time.

Keep an eye on how much leaks from the reservoir.  On my Regal, it never
held the fluid full in the reservoir, but it never went below either; never
found a leak.  I just stopped filling it and it held the lower line for a
few years before I finally sold the car.

Stop leak is BAD in a car that is NOT one step away from the junk yard.
Today's way to detect leaks is a UV dye that is added into the system your
looking for a leak in.
KIts are cheap and very effective, especially in a small leak situation.

It might have the GM stop leak already in it.

My old van came with the GM stuff already in the Dexcool,
some residue from the stuff is in the coolant overflow.

They may have only added it during the time they had so many
gasket problems.  Anyway,  your service department may or may
not know if your car came with the stuff.

He would be well advised to have a pressure check of his vehicle to
determine the cause of coolant consumption.   For instances a head gasket
that is starting to seep coolant into the oil will quickly ruin and engine.

  Might be burning coolant in the engine.   I used to add about a pint
of coolant per week to my 85 Caprice, for years and years.  Engine
never had a problem.  I did use the stop leak stuff  a few times on
that engine, and it worked for about 2 years each time.  But I think
it clogged one of the passages that cool the engine, since it
developed a "hot spot"  that caused one cylinder to "ping" at low
rpm.