[ngoma]

Not really. More likely is that your (dyed) fuel has externally dripped down onto the sensor. Remember, diesel fuel is an oil-- if it was present in the coolant you should see a oil sheen on the surface in the coolant overflow bottle. But there is no easy way for fuel to get into the coolant.

Another thought: Have you experimented shining the light on virgin coolant straight from the jug? Does it glow similarly to what you saw at the sensor?

As far as the temperature differences: How are you measuring? If you are going by the instrument panel temp. gauge that is not a reliable measure of true temperature.

Fuel economy differences could have numerous reasons.

Cups/sleeves? Please explain better.