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The best course is to siphon from the very bottom of the tank to be sure there is not a LOT of free water that has settled-out from the oil. Free water ('slugs') that comes in contact with hot injectors can/will destroy them.
Second, withdraw a sample of oil, put it into a clear glass container and hold it between strong light and your eyeball. If there is any notable 'haze' in the fuel, get rid of it.
If there is a noted 'discoloration' (not clear or clear-red; but is 'black-ish' or 'dark'), get rid of it and clean out your tank ... either by opening the tank and mechanically scrubbing the tank internals (best method); or, if the tank has no access ports by use of a chemical tank cleaner such as Starbrite "Tank Cleaner", etc.. If the tank has access ports, simply FEEL the internal walls for 'growth' accumulations on the walls; and if so, definitely clean the tank.
Recirculation polishing of the oil without a thorough tank cleaning is of absolutely NO benefit.
If all the oil-gods have aligned well - - - - no haze, no water (or you can remove the water), no 'discolored' oil ... then you're probably 'good to go' ... but still use a chemical tank cleaner to be sure. It all depends on how fouled this tank has become over time.
'Infected' tanks will require significant cleaning, etc. and yet others will retain good quality oil for very long intervals .... all depends on the oil-gods, the 'hygiene' of the tank, and the roll of the dice.
Rx: Sample the oil for haze/discoloration, check for free water, clean the tank if necessary.
Note: If there 'is' free water, absolutely change ALL the filters, including the engine mounted 'guard filter' - water over time will 'digest' and soften the cellulose fibers in fuel filters.
Small volumes of contaminated oil usually can be safely burned in a home oil burner, if all the visible water can be removed. Add a bit of 'chemical tank cleaner' to the contaminated oil and let 'sit' for several weeks or more before you add it to a home oil burner tank.
hope this helps