The Grantham score attempts to predict the distance between two amino acids, in an evolutionary sense. A lower Grantham score reflects less evolutionary distance. A higher Grantham score reflects a greater evolutionary distance. Higher Grantham scores are considered more deleterious:
The more distant two amino acids are, the less likely the amino acids are to be substituted with one another.
The more distant two amino acids are, the more damaging is their substitution.
The distance scores published by Grantham range from 5 to 215. A substitution of isoleucine for leucine, or of leucine for isoleucine, has a score of 5 (and is predicted to be tolerated). A substitution cysteine for tryptophan, or of tryptophan for cysteine, has a score of 215. Any variation involving cysteine has a high or very high Grantham score (and is predicted to be deleterious).
For more information on Grantham scores, see the following sites:
Article for Grantham R. "Amino acid difference formula to help explain protein evolution", Science. 1974 Sep 6;185(4154):862-4.