I have always kept the cross-over (sluice valves, as they are called in the Navy) closed so that the two engines would have completely independent sources of fuel; so that a wind-induced heel would no result in some fuel running into the low side from the high side (yes, it does); and so that a ruptured line on one side will not drain the fuel from both sides into the bilge. I have four nearly equally sized tanks and keep the two not providing fuel to the engines and generator completely shut off for most of the reasons above. I polish fuel from the shut-off tanks (storage tanks) into the tanks used for running (service tanks) nightly to replace the burned fuel. I would keep the cross-over valve shut if running a single-engine boat too - evening out the sides with the cross-over valve or polisher, if fitted, every time I stopped. I once moved a friend's boat a few hundred miles for him - it was a bit low on fuel and had a sluice line but NO valve. This boat ALWAYS had a list, and the fuel ran to the low side as we neared empty causing one engine to die. Arranging the fuel returns to both go to the high side solved the problem for the three hours we needed. Cross-over valves should be mandatory AND mostly closed. Rich Gano CALYPSO (GB-42 #295) Southport, FL (near Panama City)