Final answer:
Proper engine oil temperature and viscosity are essential for effective and safe car engine operation. A Carnot engine with 30% efficiency would need a high hot reservoir temperature, highlighting the importance of managing heat in engine design.
Step-by-step explanation:
The engine oil temperature for a car should be maintained within certain limits to ensure proper engine operation and prevent overheating. Viscosity of the oil is also crucial as it changes with temperature; it must be low enough at cold temperatures for easy engine start-up and sufficiently high at operating temperatures to maintain effective lubrication.
A Carnot engine with 30.0% efficiency operating with a cold reservoir at 200°C would require a significantly higher hot reservoir temperature, considering that real engines need to manage heat dissipation effectively, and most car radiators operate at temperatures above 95.0°C. This indicates that the design of engines and the selection of oil must take into account the broad range of operating temperatures.