the test can be suspended in either oil or water, that has nothing to do with an ester being present. the oil is used here so that after injection, it drains slower from the site and spreads the test crystals over a larger area. my understanding is that the reason water based suspensions hurt so bad is that the water drains quickly from the injection site leaving the crystals very concentrated in a small area in the muscle. this is just what i understood from the explanation the lab owner himself told me