Oil in Coolant Resorvoir

1997 Avalon with only 130K miles and there is suddenly about 1/8" thick layer of oil in the coolant reservoir. I'm going the bars leaks route (heavy duty), I had good luck with it on a Corolla years ago. Clearly, head gasket is very suspect, but is it possible that there is cross contamination that might not be as drastic, such as the oil cooler, or is this wishful thinking? No coolant in oil, no coolant in transmission fluid.

First things first. Is the contamination in the coolant reservoir oil (light brown?) or transmission fluid (red?). Drip some on a piece of white paper if you can't tell by looking. It's hard to believe it's a head gasket if there's no coolant in the oil. Cooler leak would also have coolant in the transmission.

Is it possible you or someone accidentally contaminated the coolant? Has the engine been overheated recently?

I would drain the coolant (including the reservoir), change the engine oil, and flush the transmission fluid. Keep them for inspection - don't dump them into a drain pan to be contaminated by old oil or whatever. Eventually, you will see the problem in the used fluids. Keep a close eye on the new oil and transmission fluid.

Forget the Bars leak.

Definitely motor oil in the coolant, a bit floating on top of resorvoir about 1/8" thick. I drained the oil and no coolant in it. I checked oil and transmission dipsticks, no traces of coolant in either. The coolant level was also low in the resorvoir. I have filled and added bars leaks heavy duty. I drove for a day and there is no sign of oil in coolant and no sign of coolant in oil.

Good luck