I have a 99 S80 that the low coolant light came on driving back from the beach. Checked and was a tiny bit low. Topped it up and came the last hour home. Checked coolant next day still fine. The following day I noticed water dripping from front passenger corner of the radiator. I drove the 5 miles home and parked it planning to see if small hose leak or radiator. Checked fluid this morning and the overflow tank is FULL of oil. No oil showing on dipstick. Car was running smooth and no loss of power. Still had smooth idle when got home. I have read some of them had an oil cooler in the radiator but I dont think this one does. Just the trans cooler. I have a compression tester but not sure if a different method to test this engine vs the ones I have worked on before which were OLD American motors. Please help.

 

I just completed the compression test and all 6 cylinders are 88-92.

 

Those pressures are with the engine cold as not able to run it to get it warm of course
seems low but it doesnt lose the 90 psi in a leak down either

 

The oil cooler on the S80 bolts to the back of the oil pan. I have seen some go bad.
That compression is to low. You should have atleast 170-190.

 

Compression tests need to be done with a wide open throttle to let in as much air as possible.
This is not a performance problem. This is an oil in the coolant problem.
It is possible for a head gasket to fail and allow oil to enter the water jacket, but it doesn't seem likely in the amounts you mention.
I think your radiator does have an oil cooler running through it on the right side.
Two ways to test.
Disconnect the oil lines at the radiator, pull the cap off the coolant reservoir and pressurize the oil cooler with about 50psi of air and check for bubbles or the sound of air escaping the coolant tank.
OR.. pressurize the cooling system with 20psi and watch for coolant coming out of the oil cooler.
If either test is positive, the oil cooler may be bypassed by connecting the oil lines with hose or a steel line capable of withstanding 150psi as a jumper
and plugging the radiator fittings to stem the flow of coolant outward.
Test first.
ED7

 

did you solve this as I have the same problem

 

I would hope so by now as this is an 8 year old post - mostly likely the poster is not active on the forum but the diagnosis process is the same - and his root cause may not be indentical to yours.