The amount allowed for imports from EU countries is 20 liters and from third countries 30 liters. This applies, for example, to travelers from Norway, Great Britain, Ireland and Belgium, where often little or no taxed diesel is available at boat filling stations.
The fuel receipt or a credible document (e.g. logbook) is sufficient as proof to customs. To date, the authorities have even imposed draconian penalties for skippers who were only caught with small amounts left in the tank. For example, a skipper who was checked with his Nauticat in Kiel had to post tax on the entire tank content of 950 liters (450 euros) and was also fined 400 euros - although after the bunkering in Norway (duty-free) and later in Denmark (duty paid) there were only traces of uncleared diesel in his tank (YACHT 17/07).
There had been bitter arguments about this between sailors and customs officers in the past. Skipper reported to the YACHT that the officials had politely advised them to drive in circles in front of the border until the tank was empty or if possible to make a pilgrimage with the canister to the nearest petrol station in Norway. There is simply no taxed diesel to be had at the bunker stations for recreational shipping.
Further information can be found at www.kreuzer-abteilung.org