I've never used a foam gun, but I've squeezed suds (off of the paint) from a foam sponge onto the paintwork for heavy soil, that is after spraying it with a hose.
Prior to using Optimum Wax, (two years ago) I was using (alternating between) M26 and Gold Class, and found that the Optimum seemed to resist dust more noticably. Ultimately, no matter what you use,...
I don't dilute it. The directions say to wipe off excess water before spraying the paintwork. Optimum Car Wax lasts me nowhere near 5 months. I use it on the S2K, which is rarely driven, and lives...