Jamie and Steve:
I respectfully disagree about additives. I limit my statements to glycol based antifrezes, I do not know about the Dexcool formulations.

First of all, at temperatures seen in the cooling system the glycol lasts forever. What does break down and get used up is the corrosion inhibitors.

So changing antifreeze on an annual or biannual basis is unnecessary
from a coolant point of view but it does replace the depleted anticorrosion
additives. I believe this is a wasteful and more costly method of getting the anticorrosion back up. Why not just replace corrosion inhibitors. It also is a lot simpler. Sometimes I have to take a bit out of the reservoir tank which I can slowly add back in the ensuing year or so.

Commercial truckers use exactly this method, they do not replace the antifreeze, the pro rad shop I discussed this with take the coolant and measure using pH strips to know how much inhibitor to add.

An alternative method is to use a DVM and Volvo according to a Volvo Tech states that more than 0.5 volts measured from engine ground to the coolant needs replacement/ or fortifying. A fresh replacement of Prestone and water 50/50%, I measured at about 0.4 Volts in another 740.

My 1990/740 at 300,000 km (180,000 miles) is still on its original antifreeze and waterpump, no leaks and on original "bad" plastic rad.
The car has never overheated and only the top hose has been replaced recently, it was getting a bit soft. Not bad for a 11 year old car (in 3 months).

I have added the combo of corrosion inhibitor and water pump lube on about an annual basis. The last time I used a slightly more expensive polymer base package from Prestone which brought the voltage down to an incredibly low 0.04 Volts. This was about 8 months ago. I just checked and it is sitting at 0.06 Volts.

I do intend to change the rad to metal and at that time I will filter the coolant and put it back in and continue to monitor the voltage.

Flame suit on!

Cheers, Sheldon
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Sheldon