Sometimes it's tough to tell if what you see inside your expansion tank resevoir is ATF or engine oil -- in fact I don't know how you can really tell, unless you pull your radiator and have the ATF portion leak tested. It doesn't take much oil at all (ATF or otherwise) mixing with your engine coolant to create the sludge you see. Thus it's tough to tell by periodically measuring your oil or ATF levels.
I actually ran my car for a couple thousand miles with the exact same conditions you're experiencing now. As it turns out I had a blown head gasket. My engine oil looked crystal clear like yours --coolant was contaminated, but no coolant entering the engine block, and it was not overheating (at least I didn't think it was). Oil contaminated sensors can give a false reading, as I found out after replacing the temp sensor sender. With oil in coolant I was getting a steady 83c temp reading at all times. After replacing the sensor, I saw the true temp readings as my engine now fluctuates between 80 and 100 depending on ambient, and whether AC is on etc...
It's certainly not good to run your car with oil-contaminated coolant, but as in my case, I got by for a couple thousand miles until I had the time to set aside to R/R the head gasket. I think the only ill-effect was that ALL rubber hoses had to be replaced. I'm not advocating you do the same, just relaying my experience
I've pulled the head twice on my 103 now, the first time being a learning experience -- 2nd time was a breeze.
I'll post a list of steps of what's involved a little bit later, then maybe follow it up with some pictures in the next day or two. This is a very doable DIY job if you're mechanically inclined, have the correct tools, and know what pitfalls to avoid.