Kubi recommends using their NarkOx oil for the rings since they say that other oils may cause the rings to swell. It's not cheap but you only need a drop rarely.
www.kubistore.com
In the US, the cheapest I've found it was:
This can be used to very lightly lubricate the Alu-Rings and/or O-Rings.
www.diverightinscuba.com
For those buying their suits from Seaskin, you can get it directly from them can add it to your order to avoid extra shipping
Use this oil to lubricate the O rings on your Kubi glove rings. Just one drop is enough. Other oils can cause the O rings to swell and cause a jam. Purchase Kubi NarkOX Oxygen Compatible Oil - 1ml Bottle from Seaskin - drysuit supplies made to order directly from the factory.
seaskin.co.uk
I don't have nearly the experience with the suit and rings that
@Boarderguy does, so if he says that spit works fine and he didn't have any problems with silicone grease, I'd trust his experience. I'm just passing along the Kubi recommendations.