Imagine taking a clear glass jar, adding 1 cup of water, and 1 cup of olive oil. You will see the oil float on top of the water. If you shake the hell out of it, you will briefly have an oil/water emulsion but it will quickly seperate back to the 2 seperate phases.
Now if you were to add a salt, or an ester (sodium chloride or test prop for instance) to the oil/water mixture, and shake it, when the oil and water settled to the 2 seperate phases, there would be an equilibrium of salt dissolved in water and in oil. In this case most (~99+%) of the salt would be in water or most test prop would be in the oil (due to reletive solubility) but the equilibrium would exist nonetheless.
One potential argument for not mixing oil and liquid based gear is that by having both oil and water phases in 1 syringe, you would have the dissolved solutes in each product dissolving in the other liquid phase as it approached dissolution equilibrium. This could potentially alter the rate at which the product is absorbed into the body and alter its kinetics.
In reality though, I do not think that the altered dissolution and absorption would be pronounced enough to make a difference in effectiveness of drug delivery, and I do not see any safety issues.
Long story short, as long as you are using sterile water, Id say go ahead and inject it.
AND OF COURSE TAP OUT ANY AIR BUBBLES FIRST!