I know, it's too soon to be talking about winter but...
I have a 50 transfer flow tank in the truck. It's the greatest thing since diesel trucks on a long trip with a 35' fiver back there. My quandary is how to approach winter. In the past I have tried to use and replace fuel in both tanks on a regular basis through the winter, but that results in both tanks almost never being full and thus leaving more air space for condensation to occur.

So this year I'm kicking around the idea of filling the Transfer Flow tank at the end of the travel season, around the end of October, and turning it off 'till spring, like maybe the end of March.

Now comes the quandary...My research on diesel long term storage has yielded results form 1 1/2 to 2 years all the way down to 28 days before some degradation starts to occur. I understand that temperature, humidity, and a dozen other factors come into play, but the bottom line is:
Just how long can I safely leave good quality fuel (Flying J) in the tank and what, if any, additives beside PM-22, should I add?

 

I think I'd run it bone dry and disconnect it till summer. Then ensure it is nice and dry next spring before fueling.

 

Is the truck going to sit all winter, or since you aren't going to be towing you just don't need the extra capacity?

If you are still using the truck I would keep the transfer tank in use. I doubt you would be able to get the tank bone dry, for that reason I would keep it full, I would make sure it didn't have any bio in it and add for Ford additive and possibly the Diesel Stabil. It the tank is metal and is going to see fluctuating temps it will create moisture if empty, and diesel fuel is prone to algae growth if it has moisture, which would be a pain in the butt to clean up in the spring.

This is just my opinion and what I would do.

 

I'd fill it to the top and stabilize it with a quality fuel treatment like Power Service Diesel Kleen. Lots of other additives out there though, and many of them stabilize fuel for storage.

 

A biocide with power service white bottle is all we use on the farm for scenarios like the one you are talking about.

You could also fill it with #1 in the event you need it.

 

I'd keep it full too... Every time I have emptied a tank it wasn't pretty months later. Everytime I kept something FULL and with treated fuel - it hasn't been an issue. I even rotate my fuel out every once in a while.

Why can't you keep both full?

What's already been said it was I would do. NO Biodiesel at all (as much as I like the stuff, it isn't a winter fuel), Power Service Arctic Formula, and even putting some #1 in it wouldn't be a bad idea either.

If you really want to empty it out I would put clean motor oil in it. And then flush it out with clean diesel before using it. The oil will coat everything and not foul the tank or lines. One of these reason long term motor storage you fog the motor with oil. I would use that type of oil too.

On Edit: You wouldn't fill it up with motor oil - just coat it and make sure oil flowed out thru the entry/exit points on the tank and thru the lines. Since this is a pain, it is why I say to keep it full and with treated fuel.

 

I do drive it in the winter, just not enough to justify the aux tank. In the past, both with this truck and my previous 6.0L I ran both tanks year around and tried to keep them as full as possible. Ended up putting just a few gallons in the aux tank to replace fuel I really didn't need to use in the first place. It looks like the consensus seems to be that a full tank with Diesel Kleen should help prevent moisture in the fuel and any problems in the Spring.

 

It is the old "If you don't use it, you lose it" saying...

 

Since you are in the North, your weather swings would concern me in the winter. A tank of Gasoline, Diesel or anything else that is not full will collect condensation. Water collecting in the tank over time would be my concern. Keep it full, stabilized and also check periodically after the truck has sat and check water in the bottom of the tank.
When I farmed in Northern Missouri, that was the rule back in the day when we ran real Diesel fuel. In the cooler and winter days and nights, make sure it is full, and always, always drain the water. Most of our equipment had sediment bowls and water was drained weekly and a lot of it, even when not using the equipment.

 

My plan will be to keep it full, fuel additive added as I always do. If you are not going to use it and keep it less then full, condensation is an issue, especially with the metal tanks. Algae will be a problem if you let the fuel sit long enough. Even with an additive.

 

Will your truck be parked inside a heated garage?

The in and out cycles will only make the condensation worse.

Mix in an additive and rotate the fuel. I.e. always fill the auxiliary tank and not the main tank.

Drain condensate as required.

 

It's parked in an open car port (roof and one wall only) so it is subject to temperature fluctuations between 0 and 60 or 65 degrees.

The auxiliary tank feeds into the main tank so, if I'm only going to use one tank, it has to be the main tank. Other then syphoning it out, the only way to rotate fuel is to burn it up. Given that the system does a good job of keeping the fuel percentage about the same in both tanks, I would need to drive about 1200 miles to completely empty the tanks. In the mean time, both tanks sit for days or weeks anywhere between 100% and 10% full. So unless I do as long trip, I'm not going to burn enough fuel to make a difference.

As far as I know, there's no low point drain to remove condensate so keeping it full for the winter seems the best option.