The parameter spread on many FETs can be quite wide... e.g. the 2N3819. There is also the 17 and 18 versions which are meant to be the "variants" of that type. I have been lucky as I have some of the original Toshiba products... e.g. the 2SK30's, 2SK369's and 2SK117's each of which have different versions (as denoted by their suffix) which have different Idss values... and thus can be chosen more accurately.
