Looking through BikeBandit by year for FXR oil tanks reinforces my statement to be sure that an aftermarket tank will fit. BikeBandit doesn't use HD part numbers but their own part numbers indicate at least some difference.
The BB part numbers for a 2000 FXR4 and a 1999 FXR2 oil tank are the same. Didn't look up the number for the 3 model, but I'm confident it's the same as the 2 and 4.
The BB part number for a 1994 FXLR, and FXR oil tank are the same, but different from the 1999 and 2000 FXR2 and 4 numbers.
The BB part number for a 1991 FXLR oil tank is different still from each of the two above. A third oil tank part number.
That's as far back as I went but again if you go aftermarket, make sure that you closely examine yours, get somebody on the phone at the vendor who can examine theirs and compare them. Or make sure you can return theirs if it's not right.