[toltion]

You can display a single Extra Field value in the product page using the following code (works in 4.1.9).

{if $extra_fields.5.field_value ne ""}{$extra_fields.5.field_value}{/if}

The number '5' in the code specifies the index value of the Extra Field. To find this value, go to the Modify Product page in admin, view the "source" HTML of the page and scroll down to the extra fields. Look for name="efields[6]" and subtract 1 from the number displayed. The index is one less than the number of the Extra Field.