Where can I find a supplier of a probe sensor for cross contamination in used motor oil.

Contamination by what fluid?

Turpentine, diesel or transmission oil, etc.

Not a expert in the field of sensor probes but I don't know of anything like your looking for. Probably what you should be doing is oil analysis.

We designed a flow through system a few years back for segregating aircraft fluid waste streams, detecting fuel and turpentine with Sn/Sn oxide sensor used in organic storage rooms (Figaro)- volatiles - and changes in viscosity was pretty simple. Similar idea - different type of detector- used in fuel dilution detector sold by Spectro for condition monitoring would also probably work. In used diesel engine oils would be more difficult for detection of low contamination since some fuel always present. Transmission fluid - unless you can pick-up dye would be very hard since compositions so similar