91 300e with oil in coolant: advice wanted

I have a "whiteish sludge" in my coolant tank (overflow tank for radiator). It may be engine oil (blown head gasket) OR transmission fluid (supposed leak and exchange between tran. fluid radiator and the coolant radiator). <p>One mechanic said it's a blown head gasket. Another said it's strange, since usually with a blown gasket you see coolant and water in the oil but not oil in the coolant/water, since the coolant/water system has higher pressure than the oil system. This guy suggested that it might be tranny fluid in the coolant due to a break in the two radiators that sit side by side. That seems unlikely to me.<p>What's the best way to determine which is which. The car runs fine, there was just some gummy white gunk on the cap of the coolant resevoir.<p>Please also copy answers to my email at: [email protected]<p>thanks!!<br>