I just reached the 3 year mark on our Extended Life Coolant, and had to add the ELC extender. Information on the bottle stated 1qt of extender for 10 gallons of system capacity. Our system capacity is 11 gallons, and I was told by the local CAT folks that 1 qt would be enough.
I was told to simply add it to the overflow tank, and when the engine gets hot, and then cools down, it will mix the extender into the system.
By the way, the CAT ELC does not use a SCA additive.
2002 3126E CAT