I also have the same problem. I recently purchased an 83 240D and the fuel light came on. The PO said that there was 2 or 3 gallons as he could get 50 or 60 miles after that.
I filled up with 13.xx gallons (17 gal capacity) and when it drained (415 miles later) the light came on.
Now, 2-fillups later, I was at 402 miles and the light never came on. I was driving to work when I completely ran out and had to coast into the nearest station. Luckily it was going downhill and the station was right at the bottom of the hill! I filled up with 17.02 gallons.
I will attempt to check the FSU as I do not want to keep draining my tank completely and I *REALLY* don't want to get stranded...