Why not oil and water together?
I've done water and an oil compound in the same shot before, no biggie!
Even know guys that do their winny and test together with no problems. If I am wrong, please explain why not?? 
For me at least, 2 reasons
1) I don't like injecting what looks like a lava-lamp into my body... lol
2) Higher risk of infection, water is not that great for sterility, mixing the two.. just seems to elevate the risk of infection
3) Like reason 1. they don't really mix together, you will have globs of water and oil all over the place, resulting in a 'not-so smooth' injection.