Does no-one on here read??
Coilovers are stock on Skylines. Just because it has an adjustable height function doesn't change that so whether they are fixed, circlip or screw thread they are "COILOVERS".
Ben, if they are too firm, or the spring and damper (shock) are not matched then you can end up with uncontrolled oscillation, or the tyre loses contact with the road over bumps (aka jumping around on the road)
So adjustable height is what some plods think is illegal, but I have seen no evidence it is.