The green ones are just silicon, they are just overpriced for no reason, i've used urethane and nitrile there with no difference, the only one that is picky is the 15/90 at the front of teh bolt, nitile don't seem to hold good enough and break/pop off, so use a urethane for that one, nitrile for the resti have sizes may see if zdspb send to UK seens as a whole o-ring kit from them is £4
and cheers scars, what are the green ones made out of 17/70 on shockers if I'm not mistaken