Don't worry about mixing it.

Some of us have done some crazy crap on the road over the years to make sure we have enough coolant to get home, having seen coke and urine poured in a radiator that had a leak, with an egg cracked into it to stop the leak (Which for some crazy reason actually worked) and seen that keep the engine cool on the highway for 45 minutes in 90 degree heat - yeah I would not worry about adding coolant to coolant!


The main reason you use the overflow valve is that when coolant is hot it can build some pressure up and if you pop the top on the radiator it can spray out and burn you supposedly. I've never seen that happen myself, but no need to open it.

 

Just so you know, the hoat from the dealer is not any more expensive than the "other stuff".

I ran prestone in my first engine but decided to go with the recommended when I put the new engine in. No idea if the non-hoat would cause any issues.

 

im a certified master technician and radiator caps do blow off but only when the system is hot, or worse when the head gasket is blown causing engine compression to pressurize the cooling system i had this happen to me last week at work after only running the truck long enough to drive it in the shop. the reservoir is for when the coolant gets hot and cold like everything else in the world coolant expands and contracts with heat so it needs a place to go hence the "overflow reservoir"
also for every psi in your system the boiling point increases by 3* Fahrenheit so a 15psi cap will put your coolant at 257*, dont open a cap on a hot system

doofy pulling hot radiator cap off FUNNY - YouTube

 

Yeah the overflow bottle was empty....already filled thanx for the info!