One way you could try is to get an oil pressure sender like this: BECK/ARNLEY 2011170. It's for a 78, and it has two terminals. One is for the oil pressure sender, and the other is a switch to ground when there is oil pressure. You would run a switched source to FPR1 and ground it at the oil pressure switch. If the engine dies, the coil de-energizes.

Another way is to use a switched source for FPR1 and include an inertia switch in the circuit.

The safest way is to combine those two ideas.

9 hours ago, anthony_c said:

"need a relay with a pull-in lower than 8 Volts"

I dare you to try that line at your local parts counter ?

LOL. Yeah, there is that. 

There are lots of relays in the world, including plenty that pull in at an appropriate voltage. But you won't find them at the local parts counter where everything is expected to work off 12V. You'll have to go somewhere like Digikey as SteveJ suggested. Or, as suggested, change the actuation source to somewhere else that would work easier with a 12V relay.

I'm not much on electrical but isn't the power of the magnet, the coil, determined by current?  Which, of course, is related to voltage.  Just pointing out that if you have a relay that's not actuating, and you're reading lower than expected voltage, getting a different relay might not be the "ideal" solution.