The only drawback I can see is if there is some problem with the additives in the automobile oil.
That's one problem. I have read some sources that indicate that some of the pour point additives commonly found in motor oil aren't a great choice to put in an item that will likely be carried and certainly handled.
The other issue is that there are additives in typical gun oils that enhance their suitability for that application--particularly in terms of corrosion prevention/protection.
You throw away a years supply every time you change the oil in your vehicles.
Used motor oil is a particularly poor choice as it is carcinogenic and has likely lost some of its original properties (why else would you be changing it?).
...unless gun oils have likewise improved.
As Gunfixr correctly notes, the basic lubricant--the oil itself, minus all additives--is likely pretty similar regardless of the specific application. Meaning that the improvements seen in cutting edge motor oils are also available in cutting edge gun oils.
I remember one debate awhile back where someone was complaining about BreakFree CLP and praising Mobil 1 Synthetic. On a hunch, I looked up the MSDSs for both and it turned out that they both used exactly the same synthetic oil (identical CAS #s) as a base.
For those who want to feel like they're beating the system by using something other than gun oil on their guns, there are better choices out there than motor oil.
According to this guy, who knows a thing or two about guns and gunsmithing, ATF makes a pretty good gun lubricant that still offers decent corrosion protection. It's not non-toxic, however.
http://www.grantcunningham.com/lubricants101.html
He recommends a Lubriplate product (FMO-350-AW oil) that is available from lubrikit.com. It's also available in a 12oz. spray container.
https://wbcindustrial.com/1-12oz-spray-can-of-lubriplate-fmo-350-aw-food-grade-mineral-oil You can also buy it in gallon quantities and split some with a friend or two or ten.
I've used some of the Lubrikit product and it works ok. I can't tell that it's better than gun oil, but it is completely non-toxic and odorless--both fairly major pluses for something that's going to be carried close to one's skin.
By the way, for those who think that all oils perform the same, here's a rather in depth evaluation of the various properties of gun lubricants/protectants.
http://thefiringline.com/forums/showthread.php?t=546316