there isnt a huge difference in a depot inject using oil or water (unless youre injecting a lot. i mean, there will be a little slower release due to the oil, but unless i see some plasma measurements comparing them, i cant really get more specific- but the difference in time for absorption for 1ml is probably an hour or two) the oil or water get absorbed by the body leaving crystals of drug, which seep into the blood and exert their effect.
the main benefit of using oil compared to water is sterility. water grows bugs. oil grows far fewer. i know many many people who went to hospital following water based injections.
if youre dying to know i could scour around and look up a study comparing absorption of oil versus water (the water will be gone in minutes, of that im sure)
ummm thats it, sorry for the boring answer