489,000 km's stil going
Sounds a lot like bardahl's stop leak product it tends to float on top of the coolant taking on an oily greenish gooy consistancy. Sometimes it also makes its way into the overflow tank.
But that stuff does work to stop a head gasket leak even before it starts. On the G2's i have heard that's an issue so it may not be so bad if applied properly and maintained with regular schedualed fluid changes.My overflow tank is filled with exactly what you just discribed. Thats why im getting a new radiatior, hoses and a new overflow tank (mine's a mess.)