I've had a 1995 850 wagon for few years now great car.
Just got a 2002 v70 t5
Noticed it was a little low on coolant. So I added probably no more than 1/2 cup if that of the ,regular universal 50/50 I've been using in my 95
Just read that this turbo uses HOAT . What I put in there was green universal. the car has deep blue in it .
Will it be ok to just go buy the HOAT deep blue finish topping off with that or should I drain it since I mixed a little universal regular green in the blue. Haven't driven more then 5 miles since doing this.
Want to do everything correct on this car

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I don't think 4 ounces of any "mismatched / chemically incompatible" coolant is enough to do any harm.

Do get your blue coolant (from Volvo is easiest) for future top offs / flushes.

I've read that one of the big, familiar makers has included G-48 in its product line. Also remember, this isn't the pale blue coolant used in many Asian vehicles.

FYI: Years ago I read G-05 is close to G-48 to the point where it's interchangeable. I switched a '99 Saab over to G-05 (after a good rinse) and it's still going strong.