The partition coefficient of the ester in question is important because is effects how long the drug itself stays in the system. If the testosterone transfers too quickly from the oil to the blood, the result is a sudden spike in testosterone which then rapidly drops once the dose has been used up. In the example of free testosterone injected into the muscle from a water suspension (as in Aquiviron, mentioned above), the testosterone is essentially immediately available to the bloodstream due to its low partition coefficient, and thus there is an immediate spike of testosterone which is used up quickly in the body.
Keep in mind that using enanthate this way will cause a significant build up of testosterone in the bloodstream that will not cease to increase until four or five weeks of injections. This is due to the fact that taking a four hundred milligram injection, and another four days later, still has at least 200mg working from the previous dose. The third injection then adds another four hundred and the first is still not entirely used up. You may realistically have over a gram or so in the bloodstream before you know it. Just be careful, and keep this in mind when figuring out your dosages.