As I recall it goes 1) Boge automatic (soft), 2) TurboGas (medium) 3) Pro Gas (pointlessly harsh). Pointlessly harsh means a stiff ride without rebound control or other benefits. Just my recollection from old SwedishBricks days. I do not know what the retaining ring is (gland nut?) and you should have the spacers in your strut tubes but it is a good idea to have a spare or pair since I have seen cars with zero, one (one side only) or two spacers. On the strut mount, treat it as a bearing to test for smoothness, and look at the rubber. Chances are yours might be coming apart but they are pricey, especially Volvo ones. When they are really bad you can get binding during tight turns such as backing out of the drive.