personally i use GM wheel bearing synthetic grease.
Will not harm rubbers
can be found in any GM car retailer(sometime you need to call and order some)
and also for the more critical , heavy stress part i use this awsome grease : MD Labs XF7 Grease
Pricey but worth it. I use it on my real steel for many years
http://www.fenixtorch.co.uk/maxpedit..._labs_xf7.html