On Jan 3, 8:51*am, Ed wrote:
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 hehttp://tinypic.com/r/34shxjq/5

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