All motor oils are great oils, and to a certain extent, all commercial oils are just about as good as any other. The problem is, a quart of motor oil does not contain a full quart of oil. All brands have additive packages which are mostly cleaning detergents, designed to scrub the innards of your engine and keep dirt from clinging to the hot metal.
As well, all of the multi-viscosity oils also have even more additives, which are there to react with the heat and expand or thicken the oil under operating conditions. A multi-viscosity oil rated at 5w-20w (a 15 weight spread) has less of these particular additives than a multi-viscosity oil with a higher range, such as 20w-50w (a 30 weight spread). When you use these oils on your gun they will work but are not quite equal to most specialty gun oils. Most gun specific oils have only one additive, and that is something designed to make the oil cling to the area upon which it is placed. Also be aware that if you are using motor oil on your gun, you are only applying the thickness (weight) of oil that is the lower number. Oil burning off the barrel not withstanding, because of the lack of heat the additive package will not kick in to change the viscosity.
To further complicate things, oils that are marketed as do-it-all products (cleaners lubes and protectants) have an even larger additive package and are therefor inherently not the best lubricants, because they are not 100% oil.
As far as rust protection, any oil that STAYS on the gun metal where it is applied and acts as an air/moisture barrier will work fine for protecting guns. I use gun oil for lubrication and protection. Cleaners have their own place.
I've seen the quiet dead; and I've heard the living moan. This world's no place to live in, but it's home.