DIESEL1. Biodiesel seems to be more prone for algae growth.2. Empty or full tank for layover:Full tank+ You have minimal area for water condensing- You have a full tank of "old" fuel in when setting at seaEmpty tank:+ You have fresh fuel in the tank with minimal moisture content (moisture damages fuel system!) but diesel doesn't go bad as fast as gasoline.- You have maximum area in the tank for water condensationI would have an empty tank if storing the boat in warm space 8-15c.I would have a full tank in a cold storage area. In cold storage the temperature is usually from -20 to +15celsius. This is to say there is ample room for condensation.About diesel additives, Volvo Penta has a bulletin stating D4/D6 engines should not use any additives. Absolutely NO GASOLINE no engine oil, no 2stroke oil, no preservatives. On older NON-commonrail engines you may use additives, but they are usually not needed.
GASOLINE
Same stuff applies for gasoline as for diesel mainly about condensation:Empty or full tank for winter period?:Full tank:+ You have minimal area for water condensing- You have a full tank of "old" fuel in when setting at sea, GASOLINE goes bad usually in a few months, definelty in under a year!Empty tank:+ You have fresh fuel in the tank with minimal moisture content (moisture damages fuel system both fuel injection BUT mostly carburetors!)- You have maximum area in the tank for water condensationI would have an empty tank if storing the boat in warm space 8-15c.I would have a full tank in a cold storage area. In cold storage the temperature is usually from -20 to +15celsius. This is to say there is ample room for condensation.Ethanol fuel:I would suggest having an empty tank in most cases when laying the boat up for the winter season. Also on carburetor engines i would preferably empty carburetors so the fuel doesn't dry up and leave moisture behind. Best seems to be using alkylate fuel when doing winterization.Ethanol fuel (E5, E10, E85, E100 etc) has ethanol alcohol content. Alcohol is hygroscopic so it attracts moisture. Then when the fuel vaporizes during off-season it leaves behind moisture that damages the fuel system. This is mostly a problem on carburetor engines that have float chambers with breather holes. Fuel injected engines are not that much affected by this when the fuel system is mostly closed, thus vaporization cannot happen.
About gasoline additives:Do not under any circumstances add oil or diesel to the fuel. Fuel treatments can be used in some cases if small amount of moisture is present in fuel. But correct remedy is to find where moisture is getting in the fuel system, usually from the tank filler or vent!Normally fuel stabilizers are not needed, but on longer periods than a few months they are worth considering especially on carburetor engines. I would suggest leaving the tank as empty as possible and filling it after a long period.
Old pushrod engines (B16,18,20,30 based and pre 1975 V6/V8) need lead additive if not converted to steel valve seats!-AQ80, 100, 120
-AQ105, 115A, 130A,B,C
-AQ165, AQ170A,B
-AQ200A,B
-AQ150
AQ115B, AQ130D and AQ170C,D have steel seats, they do not need fuel additives UNLESS somebody replaced the cylinder head with an older one!