Oil based steroids are basically oil, i.e., fat. Now if you inject fat, in some more fat, you will end up having a lot of fat. Fat as we all know has the lower blood circulation than skin or muscle. Therefore, the molecule of AAS inside all that fat will take longer to get absorbed into the bloodstream. You can also read up stuff about poisoning caused by overdose of fat soluble vitamins. They just stay in all that fat for a very long time.