so we have had one of the coldest winters we have ever had in north central il(still hanging on too, nice last couple of days, but 3-5 inches of snow coming tomorrow morning). anyway, my short trips during the winter havent changed any at all, today the temps got up to 50 so at noon I cracked the hood and opened the filler cap, to my amazement I barely had any white gunky build up at all. I hadn't even checked it since december. Last year it was TERRIBLE. like as bad as the pic above, today next to nothing. Only difference is I changed from the motorcraft oil to running Mobil super clean 5000(just their base "conventional" oil). I don't know if that is the reason for the difference, but I do know the winter this year was FAR colder and my trips were not any longer, I expected it to be really bad. just thought I'd share my observations on this.

 

My truck gets light duty too. I change yearly, usually 2000 miles or less. I have been owning/driving vehicles a long time and have always seen this 'cafe au lait' under the fill cap, and in the colder climes, even clinging to the dipstick. Water vapor is a major by-product of the internal combustion engine and it gets into the crankcase by 'blow-by', which is one reason every engine must have positive crankcase ventilation. But the outside air is also humid and that is what is drawn in to replace the crankcase fumes, so water condensation is a fact of life. And as mentioned above, the filler neck probably sits high above the engine, and in cold weather just doesn't reach a high enough temperature to cook off the condensation under the cap. So, there's no reason to freak out over it. You're changing oil every six months on a lightly-used truck, which is way beyond 'good enough'. All you can do now is to make sure the thermostat is good and its not running cold. That, plus try to make sure you get it fully warmed up each time you use it.
If you have power steering fluid, get ready for another shock when you see similar build-up under that cap. I have a vacuum canister and take the power steering fluid out of the reservoir and replace it yearly.