In U-boats the fuel floated on top of seawater which gradually replaced the diesel as it was consumed during a voyage. When fuel level got so low that seawater got into fuel lines then the Chief engineer would press seawater into that tank to drive the remaining fuel into another bunker.

When a captured Dutch U-boat was used in trials to refuel the Bv222 at sea the problem was seawater getting into the Bv222's engines, six of which could not be restarted. The Bv222 had to be towed back to port.

I rather suspect that problem was due to the small quantity of fuel available from the Dutch boat and doubt it was a real problem for a large type IX-C in the Atlantic to top up a Bv222.

Nor was the grade of fuel the real problem. Germany had a full range of fuel types available until December 1944 when Allied bombing dramatically reduced aviation fuel production. Germany produced 2,120,000 tons annually from oil fields inside it's own borders. Rumanian and Hungarian oil (6,000,000 tons pa) was never used for the Kreigsmarine and was entirely expended on eastern Front operations. Hydrogenation plants in Germany and Czechoslovakia produced 468,252 tons of diesel in 1944.

I could track down the specifications if desired but I don't think they were really the issue. I do know that when taking on Japanese fuel U-boats produced massive clouds of smoke which gave them away from miles off, due to hi sulphur content. You also have to recall some raider ships took advantage of oil from prize ships and this often included whale oils and heavy oils.