They way it was explained to me by a paint chemist was that Flat was the lowest, then Matte gloss was little higher, then Satin was next highest, then Semi-Gloss, the something called reduced sheen, then the highest or full gloss.
I wouldn't recommend the rust oleum if you plan to keep the vehicle. It might look good up front but will chalk and look pretty ugly relatively quickly. I learned my lesson on low gloss enamels....never again on the exterior parts. It ain't worth the lower cost.
If you are going to go through the trouble of painting something you plan to keep invest a little more and buy a urethane. It will hold up better and you'll be more pleased in the long run. These guys make a nice one I have used and it is affordable. They made a special color for me and it worked well. It seems like they said they could do any gloss range for me also.
The best approach is to use a pre-flattened urethane clear over a urethane basecoat but a single stage urethane flat works fine for me.