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Plastic valve covers and intake manifold on the Fiesta, as well as many of the newer cars. Much less expensive to manufacture than metal castings. As far as long term reliability, that remains to be seen. When you consider the total surface area of the engine available to radiate heat the valve covers are a small percentage.
Valve springs would not overheat until the coolant temperatures were astronomical. Oil cooling for the valve stems, with the valve heads radiating heat into the cylinder head. The seals would probably melt first and then the engine would start smoking like crazy.
Of course driving for mileage means lower overall temps compared to design maximums so I see no problem with insulating a lot more than just the valve covers if you are in lower temperature climates.
Get down to 20 below and you could probably spray insulation over the whole engine and just leave the oil filter exposed to a flow of minus 20 air.
Just remember the engine is acutally cooled by air, oil, and water-antifreeze.
regards
Mech
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