ABI recommends to use the HiDi formamide for loading. Some labs use Milli-Q water instead, but in my experience resolution of sequence peaks was inferior to formamide. The HiDi can be replaced by some high quality formamide. I have tested formamide of molecular biology grade from Sigma (F-9037) and found it is compatible with ABI3100 for sequencing run. It is stored in refrigerator for about one month, and can be frozen in aliquot for longer storage.
It does not work as is, however, for 4-color fragment analysis run because of low signals. Addition of 0.2uL of 10x running buffer to every 10uL of formamide improves the signal strength to about a half of that with HiDi.