Sorry to get technical here. Dexcool is based on 2-ethylhexanoic acid + sebacic acid with a dash of tolytriazole. It is the same (or close) as the blend used in Peaks Long-Life & many heavy duty (semi truck/crane/ship) coolants. The pH of Dexcool is about 8.5 to 9 (very slightly basic. It is known as "OAT" coolant (organic acid technology).


Conventional (IAT-inorganic acid technology) coolant (what USED to be all green & the only coolant available for our cars when they were built) contains silicates, phosphates, borates, etc. and typically has a pH of about 10 to 10.5 (pretty basic). These ingredients form sludge when the pH gets below about 9.5, so if you topped off DexCool with conventional coolant, sludge would form.


Based on personally analyzing a lot of coolant from original owner cars whose owners swear nobody has ever worked on their cooling system, changed coolant, etc I can say that just about everybody's coolant has been "topped off" by somebody, somewhere....like Jiffy Lube or wherever.

And on a related note....2-ethylhexanoic acid can dissolve some gasket & o-ring materials. This seems to have been taken care of by the gasket manufacturers since "dexcool-type" technology is still widely used.

Side note...Prestone's "green" (actually yellow for Prestone) is also an "OAT" formula antifreeze, but is not based on 2-EHA. Prestone's yellow & Peak's green are compatible with each other too, fyi....and so is Dexcool. The closest thing I've seen to the OEM-type coolant is actually called "Peak Green" & comes in jugs which look very similar to their popular "long life". It's conventional, based primarily on phosphates (high pH, not compatible with Dexcool, Peak Long Life or Prestone green/yellow). I've seen it behind the counter (you have to ask) at some O'Reilly's