I was prepping for a trip down into the SanRafael swell this weekend and checked the engine oil. Little low so I decided to top her off. When I pulled the oil filler cap there was a reddish rust color film on the underside of the oil filler cap. This is the first time I've had to top off the oil since I changed it in May of 2008 about a 1000 miles ago. I cant say how long or if its always been like this. Is this a sign of a head gasket issue or just condensation. What do I need to check to verify I dont have an issue?