Mine is a sure-grip. The Mobil 1 bottle implys you do not need to add sure-grip additive but I put in 1/2 a bottle with the Mobil 1. I have no problems with shuddering.
You can mix all Mobil 1 synthetic products with conventional lubes, you just loose most of the advantages of having a syntetic lube. If you just want to top up for one more race I would just truck down to Wal-Mart and get what ever's on the shelf.