On the GCP and Mini CP after some expirementing I have found that the #005 Buna 70 orings from oringsandmore.com to be a good compromise between too little and too much tension for the arms.
I also have some Buna 90's #005 but am currently stricken with a back problem that only allows me to lay on my back. Am awaiting back surgery and will test them the moment I can get off this danged floor.
At the cheap prices you may want to get several sizes to try out as I did.
As always, YMMV