Continued with wiring.
Connect master switch. Main buss is now switched.
Connect oil pressure/master-on sender and LED. For this I use the Van's Hobbs oil pressure sender. 12V is hooked up to common, and an led to the NC connector. When there's no oil pressure, the red LED comes on. It has the extra duty to be on when the master switch is on, reminding me (hopefully) to turn it off at shutdown time. No need for a push to test, just turn on the master.