Sorry I didn't get back earlier. It should be easy to check for transmission oil contamination by checking that your trans oil on the dipstick is clean. If so, then I suspect it is engine oil. I read in an earlier thread that if the heat exchanger in the radiator leaks then due to differential pressures there is always 2-way contamination i.e. you will get oil in the coolant and coolant in the oil - this appears as emulsification of the trans fluid.
In my case I bought my 300TE-24 and noticed a small amount of brown sludge in the header tank. The car had fairly recently had a new head gasket fitted and I assumed that this was just some residue from that (it showed no other signs of head gasket failure). A few days later I checked the trans oil and found it emulsified. The trans at that time was working fine. I checked the radiator which was fine - new in fact! I later found out it had been replaced the day I collected the car without the PO's knowledge when it had been in for an a/c repair. I changed the trans fluid 3 times and now it is playing up due to damage done by the coolant. I am faced with having it replaced or rebuilt. Lesson learned - always check trans fluid before buying! I should know better at my age!
That's my story. I hope for your sake it's your head gasket - much cheaper than trans!
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)