My advice, considering all the good information posted already, would be to grab some StarTron tank cleaner. It breaks down a lot of the gunge that builds up in the fuel tank. A benefit of that is that the guide wires for the fuel sender will be cleaned somewhat and give your fuel needle a considerably more accurate reading.

Fuel Sender 85 300D- Cleaned (Pics)

Star Tron

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1977 Red 240D ( Squished by a 18 Wheeler )
1976 Black 240D ( Sold )
1991 White 420SEL ( Sold )
1987 Black 560SEL ( Sold )
1983 White 300SD ( Sold )
1984 Blue 300SD ( Sold )
2014 Red Chevy Cruze Diesel ( New Hotness )