Clean the saw completely free of oil residue and fill the oil tank- sit it on a sheet of clean cardboard and pin point where the leak first appears- you have to know where it is leaking from in order to attempt to fix it.
Could be a clogged or faulty tank went and use forms a vacuum and oil continues to syphon through the oil delivery system until pressure equalises or the tank runs dry.
If you track it down to a joint or hard to find/replace part, perhaps it can be "woods fixed" with some liquid gasket maker?
But first you have to figure out exactly where it is leaking from so you can repair that leak- not just throw parts at it and hope.
EDIT: Poulan saws were not that common over here, so I am not dead familiar with the model, but is an 1800 like a 2500 Micro and runs a pressurised oil tank, uses crankcase pressure to force oil through two check valves to the bar delivery gallery?
If so those two valves would be my first port of call- the one that screws into the crankcase has a duckbill inside (that may be rotted out) and a sealing washer not unlike an automotive sump plug- may need a new washer.
The other is on the opposite side of the oil tank from the filler and held in by two screws- also has a check valve in the form of a ball and spring that could be stuck open, but may need a new gasket made between cover plate and oil tank.
