Your situation sounds perfectly normal to me. 370Zs tend to run on the warm side. Stop and go traffic makes things worse. Car will limit power if oil temp gets too high. Get an oil cooler if the "high" temps bother you.
The loss of power in the afternoon may be due to high IAT. IIRC, the car starts limiting power in the mid-80s degrees F. Most OBD readers will show IAT.
FWIW, most oils are good up to 300 degrees F. Not that you'd want to run the engine that hot, but the oil would take it.
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