Either condensation or someone put it there. Diesel can get into the coolant, but it won't happen the other way around.
Depends what you want, power, economy or low $$$$ For power, go with a CAT For economy Detroit S60 Dor low $$$ Cummins.
The 2006 Volvo S60 has an inline 5 engine.
The 2001 Volvo S60 has an inline 5 engine.
The 2002 Volvo S60 has an inline 5 engine.
The 2004 Volvo S60 has an inline 5 engine.
The 2008 Volvo S60 has an inline 5 engine.
The 2003 Volvo S60 has an inline 5 engine.
The 2015 Volvo S60 has an inline 4 engine.
The 2007 Volvo S60 has an inline 5 engine.
The 2009 Volvo S60 has an inline 5 engine.
The 2011 Volvo S60 has an inline 6 engine.
The 2014 Volvo S60 has an inline 5 engine.