After you have a thousand miles on the new oil, send a sample to a lab to see if you have coolant in the oil.

Entirely possible that the loss of coolant is external rather than internal.

If you don't see it leaking on the ground, check the heater core. See if you can sense wetness near the gas pedal.

Regards

Don't forget the other hints, like checking the water pump gasket and the coolant cap for cracks. A new cap is cheap.

Foam on the dipstick is usually from condensation when the oil never heats up to cause the water to evaporate. Your oil change removed all of the water in the crankcase. This takes a few weeks of short trips to build back up.

Klaus
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The 164 has a new home, all I am left with are a 95 854T and a 98 V70R :)

My experience matches the other post -

1 - external coolant leaks due to loose clamps, etc. aren't rare. Look under and behind your timing cover for leaks at or near the water pump.

2 - particularly on cars often driven on short trips there could be condensate in the oil along the dipstick or under the filler cap. Many have reported this.

3 - is there oil in the coolant - for instance floating on the coolant in the expansion tank, when the engine has rested cold for many hours? If there's a head gasket leak it should go both ways.

4 - if the coolant is contaminated you would want to eliminate that it is transmission fluid entering thru an internal defect in the radiator.

Is your oil significantly different than prior to the leakage? Lots of Volvo's have this issue, and it's not a head gaskey, so don't assume this is the problem.

My head gasket went a year ago, it's not a repair for the faint hearted. On a scale of 1-10 it is about a 9.

Start by getting your coolant system pressure checked, if you are lucky you have an external leak. A leak from the rad to the oil cooler should result in oil in the coolant, not coolant in the oil, although both are possible. At the least you should see cross contamination.

The worst case is the head gasket, with a pressure test and the plugs pulled you might see/hear a leak. Depending on the value of your car this may not be an economical repair, unless you can do it yourself.

I was prepared to scrap my car, but when I pulled the head I found that the cylinder bores and the head were like new, even at 160,000 miles, so I fixed the car and replaced the oil trap, along with the vacuum lines. I still get condensation and some froth on the dipstick from time to time.

You need to fix this, don't drive much until you fix it.