I am not sure if intramuscular injections are slow release. Anyway I was injecting my hydroxocobalamin subcutaneously.
In any case, the B12 doses you get from a vial of injectable B12 are typically in the 500 to 1000 mcg range (and are only one form of B12). Whereas one shot of the B12 oils provides around 2000 mcg of systemically available B12 (and provides two forms of B12 in the same shot: methylcobalamin and adenosylcobalamin).
It's probably a good idea to also take the cofactors recommended by Greg (which are all to be found in this supplement that he suggests:
Life Extension's Two-Per-Day Multivitamin and Mineral).
And I found the shot of the B12 oil would often cause a temporary wave of tiredness a few hours after rubbing it onto my skin, this tiredness lasting for about 4 hours. But the tiredness is prevented if I take around 400 mg or more of oral potassium with the B12. I believe the potassium helps prevent the hypokalemia that B12 can cause. So consider potassium if you experience this temporary increased tiredness. You get about 400 mg of potassium in a banana.