I have a problem with a GM 3400 SFI v6 in an Oldsmobile Silouhette. I have oil residue in the radiator overflow tank, other than this it shows no signs of oil in the radiator. No signs of coolant in the oil. No smoke from the tailpipe or leaks. Engine does not run hot and fuel economy is holding around 20 MPG. I am thinking it is the intake manifold gasket. Have you heard anything like or know of anything I can do to confirm. Thanks for the help.