system
Given the following post, respond as if you're commenting on Reddit:
human
Is my car engine more efficient on hot or cold days?

Let us define "Hot" and "Cold" to be in the vicinity of 80^o F and 30^o F respectively.
gpt
It depends on how long you're going to be operating it for, and if you live in the real world.

Cooler intake air will be more dense and have higher mass flow per piston-cycle. Getting more mass of oxygen into the cylinder will allow more fuel to be burnt, making each cycle produce more power (but also use more fuel). Cooler intake air also reduces knock (premature explosion of the fuel-air mixture due to heating via compression).

Cooler temperatures will also make your various lubricants more viscous and thick. Proper engine oil shouldn't be affected by this at such mild temperatures listed in your query, but can be affected when things drop below -30 centigrade. Oils are engineered nowadays to have the same viscosity within a wide range of temperatures. The real losses come from places that are lubricated with grease, like wheel bearings. These warm up relatively quickly though.

Starting out, your engine block will be cold, the intake air will be cold and the exhaust gasses will cool rapidly within the cold exhaust ducting. Rapidly cooling exhaust gasses is unwanted since you'll observe greater back pressure. This reduces engine air flow, and thus efficiency. All of these systems warm up as you drive. As they warm up, your efficiency will increase.

So, if you're driving 5-10 minutes to the shops, your efficiency will be worse.

If you're driving 30 minutes on the highway to work, not in stop-go traffic, your efficiency will be better at cooler ambient temperatures than warmer.

Carnot efficiency (as outlined in another reply) is only a small part of the equation.