Thanks to all of you who replied to my previous posts. The stuff in my coolant overflow tank is black, not red, as I would expect ATF fluid to be. The oil in both the engine and the trans does not appear to be milky or diluted. However this goo is getting into the cooling system, it does not appear to be a two way transfer.

So, if I understand it correctly, this stuff in the overflow tank could be:
1.) Oil, from either a leaking head gasket or an oil cooler (even tho I don't think the oil cooler and the radiator share a common line, right?)?

2.) ATF from a leak in the radiator (although this should be a red colored fluid, right?)

3.) Oil from a "one way" head gasket leak?

So, how do I tell what it is? What tests should be performed? Any advice is much appreciated.

Thanks!

Gavin

1990 745T
1976 2002
2002 330i