I have a volvo S60 T5 auto 2001 model and when I was doing oil change I noticed a chocolate like colour on the oil catridge holder. Checked the oil filler cap and had a similar chocolate colour. In the expansion bolttle they is a sludge like colour.The car did not over heat as far as the temp gauge was indicating. When draining the sump the water level in the expansion bottle dropped. Removed oil cooler and pressure tested ok. No water in the gear oil. What could be the problem?

 

If it is not the oil cooler I would say headgasket.