Stupid question maybe?
How do you hook up hour meter so that that it logs hours when the motor is running instead of when the ignition key is "on"?

Thanks in advance.

1 idea is to tee a Hobbs switch into the oil pressure sender. Wire the Hr meter through the switch and when the switch detects pressure it will complete the circuit.

Most standard setups have both an analog oil pressure sender and a switch also. Only trick would be to wire it with a dual-pole relay so that when the oil pressure switch was not alarming it would complete the circuit and power the hour meter. As long as the supply to the line side of the relay came from the ignition, it wouldn't run when off.

I just pull the connector to the hour meter whenever I have to do any key-on maintenance.