OK my first post went nowhere so I'll simplify the problem and then try to expand on it later.

Essentially I'm looking for a way/probe that can indicate when I reach the interface of a binary liquid system (ie oil and water).
The probe will be lowered through the top liquid/layer and would indicate when the probe enters the lower liquid. Of note the top liquid has a much lower resistance than the lower liquid.

Any help would be greatly appreciated !

 

Are you saying water floats on your oil or you have electrically conductive oil, or something else?

 

I'd approach it not from the electrical conductivity angle, but the dielectric constant.

Here's a page with lots of dielectric constants:
http://clippercontrols.com/pages/dielectric-values#O

The dielectric constant of a perfect vacuum is 1.
The dielectric constant of air at sea level isn't much higher; something like 1.0000001123 or thereabouts.
The dielectric constant of oils is in the 2 to 3-something range.
The dielectric constant of water at room temperature is 80.

So, if you had a couple of parallel metal conductive plates forming a capacitor, and they were used as part of the timing circuit in a free-running oscillator, you would see a sudden drop in frequency of the oscillator when the plates had water in between them instead of oil or air.

The plates should be electrically insulated from the liquid. A thin coat of lacquer, conformal coat or epoxy paint should be sufficient.

 

I solved my problem

Unfortunately capacitance approach was not an option due to the extreme temperature and corrosive nature of one of the liquids. I simply used a voltage divider with an LED in parallel as an indicator. When I'm in the higher resistance liquid the LED is on and when I move to the lower resistance liquid the LED is off.
Extremely simple and it works !