You will probably be very disappointed and will not like the performance of a DP level transmitter for a mixture of oil and water.

DP measures head pressure. Level is inferred from head pressure by a compensation calculation using the density of the fluid. If the density changes, the indicated level changes for the same head pressure measurement.

It is very unlikely that the mixture will always be the same percentages of water, oil and gas. The mixture ratios will change over time, the density will change and the level readings will never be correct.

Have tried Fluid Components International in San Marcos?

I agree with Iris if the density can change the level measurement will not be very accurate or realible.

Some manufacturers use a flexible weighted rope type capacitance probe that can easily be removed with limited head room.

YOU can use radar type level transmitter.