How to turn on an LED when voltage drops to low?
An Arduino, at a cost of under $5, is an excellent solution.

This circuit should work, but values will depend somewhat on the oil pressure sensor resistance. 

Reducing sensor voltage below a certain level will turn on Q1 and so LED1.
R2 can be adjusted to vary the turn on level. 

The circuit can be improved if of interest. 







simulate this circuit &ndash; Schematic created using CircuitLab