gauge on fuel oil tank went dark

for my oil-fired boiler, the fuel oil tank in the basement has the see-through plastic fuel gauge on top. It's cylindrical, vertical and marked off in quarters. Approximately the bottom quarter of it went dark for some odd reason, as if stained from the inside. The boiler still works the same as usual. It was a very rainy year. The tank was filled last spring but I don't think that this problem appeared right after that, but I'm not 100% sure of that.
Any ideas what did it?
photo here: http://tinypic.com/r/34shxjq/5
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I used to rent a house that had an oil furnace, the level indicator was a clear tube that ran from the bottom of the tank to the top. The bottom portion of the tube also turned dark.I think the fuel just eventually stained the plastic tube. It seem logical because one tank full usually last the winter and would not be filled again until the beginning of the next heating season. The portion that was stained always had fuel in it.
Jimmie
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On 1/3/2012 8:51 AM, Ed wrote:

Unscrew it, and see if you can clean it. If not buy a replacement. Here is a link to a "king" (Amazon.com product link shortened)
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On 1/3/2012 3:59 PM, RBM wrote:

I'd say the same thing although unit you refer to does not look the same.
Thread prompted me to look at mine, which is similar, and, I found would unscrew easily and cleaned up with a damp rag. What was making mine cloudy was on the outside.
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Typical of very old oil level gauges. The PVC cylinder is always exposed to fuel vapor, and the plastic decomposes a bit, releasing (probably) micron sized carbon particles which look brown in the light. Consider replacing the gauge with a new one if the rest of the system is up to it.
Joe
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On 1/3/2012 5:51 AM, Ed wrote:

I suspect the dye in the oil is doing it. Since fuel oil can be used in Diesel engines on the road, it is dyed to indicate no road tax was paid on it. If used in a road vehicle, the dye will be proof of non payment of the tax. At least that is true in Oregon.
Paul
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