Oil cools and contracts, collects in an oil-pan, etc.
I have adopted a method with my 'vette that is more strict than most because of the dry-sump.
I check the oil at EXACTLY the same temp/time after shut-off every time. If I do not, the variance can be huge. As in, a quart, easily.
With the regular cars I have owned, the difference was usually 1/4-1/2 quart or so.
I suggest checking the car 1 hour after shutting it off from running it with the oil at a certain temp.
Record temp and level.
Then next time you check it, make sure that you shut it off at roughly the same temp (+-5*) on a similarly ambient temp. day, and wait the same amount of time and check it.
Make sure you part in the same spot in the same orientation.
This should eliminate any variables attributed to anything but consumption/lack thereof.
*With a dry sump system, you are to wait 5-15 minutes, not an hour, but I presume your Z has an OEM oil system. I would wait 1 hour to let a decent amount of oil drain back into the pan.
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