My :2cents:, Richard:
I don't believe you'll have much condensation, if any, if your stored fuel is allowed to warm/cool SLOWLY - it shouldn't cause the moisture/humidity to condensate out of the air within the container and contaminate your fuel. By storing your fuel storage can/drum/tank inside under a roof will keep the Sun from warming the fuels excessively and will also prevent it from cooling off quickly in the evening under a clear star-lit night. Even a tarp will be better than nothing in shielding the storage vessel from extremes in temperature change. Central TX isn't a high humidity area...so if you store your fuels out of direct sunlight, I'd bet you'll be fine regarding moisture contamination. I used to live in Houston; now that is a high humidity area!
Here is another way to look at it: Consider when it's been cold for a week or so and then there is a sudden increase in the outdoor temperature. If you open your barn or garage door, the cool metal & concrete inside will begin to sweat. This is the caused by the moisture in the incoming warmer air to condensate onto the cooler metal & concrete. However, if you keep your doors closed, and let the garage warm up slowly, the stuff inside will not draw condensation from the warmer air. Obviously, if you keep your storage can/drum/tank full you reduce the amount of air inside the vessel and thus the potential for moisture-ladened air from being inside.
However, the above discussion only is addressing moisture contamination via condensation from the surrounding air. You will still have other issues going on - namely, the lighter hydrocarbon ingredients will slowly evaporate out and leave your fuel with a disproportionate amount of heavy hydrocarbons...in other words, the mixture that makes up your diesel fuel will be off. This can be made obvious by simply splashing a few ounces of diesel fuel into an empty coffee-sized can and leave it sit on the shelf for a day, week, month, few months. You'll notice how the odor of the stored fuel will gradually change. This is because some of the more volatile ingredients will evaporate away, leaving only the heavy ingredients to produce the remaining odor. We've all heard of fuel "turning to varnish" after some time...... that is what is the result of the long time storage......
Bottomline: Unless you can keep them in a container without any venting provisions - keeping moisture-containing air out and keeping the light hydrocarbons from leaving - I would recommend you store diesel or gasoline fuels no longer than 6 weeks. Realize though, if heated enough, pressure will be produced within a non-vented container, so keep the container relatively cool....less than 80*F, to keep from producing excessive container pressure. You can easily check the temperature of your fuel storage vessel with a temperature gun. Like this one:
Sears.com I wouldn't worry as much about your oils & gear lubes, as they are a much heavier fluid with small amounts of light hydrocarbons. Most drums/pails/jugs of oils & lubes do not need to be vented (if kept cool) and thus will not become contaminated or loose ingredients over a few months time.
BarnieTrk :steeringwheel:
P.S.
Richard, you do run coolant and not straight water, correct?
I knew of a few folks in Houston that ran straight water; with the claim that, "It doesn't get cold enough for me to worry about freezing" - an opinion which I did not / do not agree with.