Blackening steel with oil vs. vinegar - and others?
Treating with vinegar (or other acids) tends to form a dark grey patina, depending on how it is applied it can also also show variations in shade. More viscous things like mustard and tomato sauce can be dabbled or stippled on to create a distinctly mottled effect. Acids also work well to strip off or at least dull bright zing plating and galvanising. 

This finish needs to be dried and sealed with wax, oil, lacquer etc fairly quickly (ie within minutes) or it will start to rust. Also because it is somewhat porous it can be a good base for tinted oils and waxes. 

Oil blacking works by polymerising and partly burning the oil. Here the temperature is quite important as the metal needs to be hot enough to blacken the oil without burning it off completely. If you watch the oxide colours as it heats up the point when it goes from deep blue to grey is usually about right. This process is pretty similar to seasoning a cast iron pan. You can either dip the piece in a container of oil or paint it on with a rag (don't use a synthetic brush, it will just melt). 

Oil blacking is reasonably stable and doesn't require any additional sealing, although a layer of hard wax, reapplied every so often will make it a bit more durable. 

I would recommend using linseed oil or similar rather than motor oil as motor oil (especially used) will contain all sorts of additives that you don't really want to be handling or inhaling. I've found that a mixture of linseed oil and paraffin wax works well.

Another option is to allow the piece to rust and then treat it with a rust converter such as Fertan, these are generally phosphoric and/or tannic acid based and will turn the rust a deep blue/black which can then be sealed with wax, oil etc