I knew of one plane, a twin...it had the hobbsmeter on one of the two engines.
We always shut that one down to practice our single engine work, using our wristwatch for logbook time, hobbs time for billing.
AND THEN we figured out, you could just pull the oil pressure circuit breaker and turn the hobbs off, again using your wristwatch for logbook time.
But you always worried if you would lose oil pressure.
But then again, I might just be making this all up and you should never try it in real life!