Synthetic Oil is 100% compatible with Conventional/Mineral Oil, you can switch back and forth or even mix them together. (But its NOT recommended to mix different oils, NOT because they may be incompatible, but because you less likely to create a better special formula than just an oil already manufactured).
One note, most have already touched on it in other threads. There are two kinds of "True Synthetic Oil".
Castrol started marketing conventional/mineral oil that under went a highly refined process called "Hydrocracking", that does modify the molecule and is loosely a synthesizing process, as "Synthetic" and was sued by the other Synthetic Oil manufacturers. The courts decided "Synthetic" is just a marketing term and as long as there was some sort of synthesizing process, you can call it synthetic.
So, to rub it in after the verdict, Castrol actually had the gal to put "True Synthetic" on bottle, and sell it for the same price as the other "True Synthetic"'s despite it costing half as much to produce. So its NOT hard to imagine the other manufacturers saw this working so well with such a high profit margin and went the same way.
And before you get really upset if you've been using Castrol Syntec or others, like Penzoil, don't worry, the "Hydrocracked" NOT so "True Synthetic" oil is still superior oil and much better than other conventional oil, your engine was well protected.
Group II Oils are the typical conventional oils you buy in the store.
Group III Oils are the highly refined "Hydrocracked" oils, they are conventional oil base stock, they are better but they are NOT as good as the next group.
Group IV Oils are what people think of when they think of "Synthetic" oil, these are PAO or other fully synthesized base stock, that the molecule is constructed from the ground up with basic chemicals. Group III produces their result working backwards, taking the conventional molecule and modify most of them close to the ideal molecule.
Mobil 1, its been a big debate if they have gone the way of the others and use Group III oils for base stock. Most believe they have NOT and Mobil 1 is still at least 70% PAO base stock. The more exotic brands, like Royal Purple and AMSOIL are PAO base stock. (Note AMSOIL has started selling a cheaper line of motor oil, OE and XL, that most believe is Group III base stock, and they seem to admit it with a disclaimer on the bottle that it is their less expensive and NOT as long lasting as their other products).
Readers Digest Version:
Get Mobil 1 in the 5 qt Jug from Walmart, its barely more than good conventional oil, it will do great in your Commander and just about any other vehicle that recommends the current API certified motor oils.