Are Stackable Poly Tanks U/L approved?

No.  To receive the proper U/L Rating (U/L 142), Tanks need to withstand a fire test.  Obviously, poly tanks melt when exposed to fire.  Some manufacturers of stackable poly tanks claim to be U/L approved.  This is a sham.  They are not properly U/L approved, and if you plan to store Class IIIB flammable products in them (Waste Oil), you’ll get cited.