Take a look at the oil and see if it is the right color. Dark not brown. Check the end of dip stick and under the coolant tank cap for sludge. The oil coolers/o-rings can sometimes fail. Also, check under the trunk carpet and see if it is wet.
Search through Pelican's tech articles, lots of good free info;
Porsche Boxster 986/987 (1997-2004) Technical Articles | Pelican Parts