Originally Posted by jmg14213

That's not entirely true. When you turn the engine off (autostop), the water pump stops as well. However the cylinder head, spark plugs, valves, pistons, and block all have excess heat that needs to dissipate. That's why the fan comes on AFTER you turn the key off, and it also comes on if the autostop has been active too long. There will be some convection cooling in the water jacket and radiator, and the aluminum block will also shed some heat from the air flowing across it. Due to the "heat soak" problem on these engines, stopping and starting all the time, I am surprised that Honda hasn't specified that the oil must be synthetic, regardless of whether it is 0w20, 0w30, 5w20, or 5w30, just for the better high temperature properties of synthetic oils. Also, the synthetics more closely maintain their viscosity ratings.

Also, the smaller engines in our hybrids are not really any more efficient than similarly designed large engines relative to their displacement. And more efficient engines have to run hotter, since all internal combustion engines operate on the principle of thermal expansion of gases. The greater the heat rise, the more expansion and combustion pressure to power the pistons down their cylinder bores. What makes any hybrid so efficient is that with a reasonably aerodynamic vehicle, cruising uses very little power. I read once when GM brought out the Chevy Citation, it needed 12 horsepower to MAINTAIN 55 mph. It is getting up to 55 and being able to accelerate for passing and such that requires all the extra power. Our hybrids use the electric systems to provide the power for acceleration. The gas engine is barely larger than needed to keep the car moving at constant speed. That's also why your fuel economy is so crummy in stop and go traffic, even with autostop and boost. In fact, our little engines have to work their pistons out to climb hills or accelerate past that slow semi on the freeway, resulting in the CVT revving the engine all the way to redline, or the downshift lights in the manual trans recommending third or second. All that hard work also makes them run a lot hotter. If you've ever watched your temperature guage, it gets up to "normal" really fast in the morning, even in winter.

The main reason for the 0w20 is that 0w20 weight oil uses less power to pump it up through the engine. It also flows more easily into the tight tolerances Honda and Toyota have designed into their engines. This is the same reason that all new GM and Ford vehicles recommend either 5w20 or 5w30 oils instead of the older 10w30 or 10w40 from years ago. The lighter oil uses less power, triggering better fuel economy. It is true the improvement is minor, but every 0.1 mpg counts when you're selling 500,000 cars and they have to reach a certain average fuel economy for your company. What with all the trucks and SUVs, it is amazing that GM, Ford and Chrysler can even meet the current C.A.F.E. standards.