In the latest IPD catalog, they include on page 7 a "Tech Tip: Fluid Changing". In particular, they discuss a method to check for acid buildup by using a voltmeter, grounding one probe and inserting the other lead into the coolant reservoir so that the tip of the lead is immersed in coolant. They indicate an ideal reading is 0.1 volts or less while anything above 0.3 to 0.4 means the coolant is ready to be changed.

I tried this tonite and found that my wife's 99 V-70, where I used "expensive" Volvo coolant about 6K ago had (I'm guessing cause my crappy Radio Shack guage scale was 5v) about 0.1 volt. The reading on our 93 244 where the coolant was changed with aftermarket antifreeze about 15K ago read significantly higher (I'm estimating) about 0.4 volts.

According to IPD, corrosion protection is a key feature of Volvo coolant which minimizes acid buildup. I know there's been much discussion about coolant concerns here on the BB, but I wanted to bring it up again. If what IPD says is true (not that I'm disputing them), and the aftermarket coolant was played after (maybe even sooner) just a year of use; this means I've learned another important lesson.