Have you checked your trans fluid on the dipstick? If the oil was clear then it is not likely to be the heat exchanger in the radiator. As I understand it due to differential pressures you will always get 2-way contamination. If you want to be sure disconnect the trans fluid pipes at the radiator when you do a pressure test. Clean the connections before disconnecting, cap the pipes and ensure you do not contaminate the fluid. Any leakage should show at the radiator. Obviously dont run the engine with the pipes disconnected and remember to top up any trans fluid loss.
In my case the oil in the coolant was brownish though, strangely, the transmission fluid when drained was colourless, just emulsified.
My money is still on the head gasket though.
Good luck
Adrian

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90 300TE 24v (M104 eng) 207K miles
87 300TE (M103 eng) 269K miles (and never had the head off!)
86 300TE (M103 eng) 230K+ miles (donor car)
95 Toyota Lucida diesel 4wd
86 200T 165K miles (sold)