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Adrian, @Martin's Eleven (UK) is right about a number of things.
The way you phrased your question had us making suggestions for good engine bay ventilation, but proper oil cooling is just as (and maybe more) important.
The stock VW engine never made much more than about 50 hp, so the stock (doghouse) oil cooler was adequate in most conditions.
But most of our engines make double that power or more (some a lot more), so external oil coolers with electric fans are common additions. These engines are also very sensitive to ambient temperature. I can pretty accurately tell what the outside air temperature is by watching my temperature gauge. Below 65 degrees, the fan on the cooler is almost never needed. But above that, it is.
An external cooler can cause problems of its own in colder weather, though. If it's plumbed permanently 'in line', the engine can take forever to get up to proper operating temperature. So, most of these are hooked up through a thermostat that routes oil back to the engine until the oil gets hot enough.
Again, it depends on how much power your engine makes, where you drive, and how you drive, but external coolers (and extended sumps, too) are often required when running high performance VW engines in the cramped engine bay of a Speedster.
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