What does the "lightly" shaded part of the tach represent? Roughly between 3,000 and redline.
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My guess is its the operational engine RPMs. Slower than 3k and you're lugging the engine or nor spinning the fan fast enough for proper cooling. Guess you'd need a real 356 Owner's Manual to verify.
It's an indication of the power band of the engine (Along with Wolfgang's idea of the proper working range for the cooling fan).
These engines will "lug", i.e. shudder a bit with accompanied pre-ignition knocking, when power is requested well below the working power band.
The VW engine designers knew this, so in order to keep the engine well within the power band (which starts coming on around 2,500 rpm) they set the indicator on the Tach starting at 3,000 rpm as a Green highlighted band (On some tachs it has separate lines for each gear, but that's rare) which terminate at the redline.
VW sedans didn't have a tach, so they had separate green lines for each gear on the speedometer instead. Both served the same purpose.
Through my reading (actual paper books and manuals) I thought that was what they indicated so I drive around town mostly in 3rd gear to keep the RPMs up into that range. Thank you for verifying my speculation.