I thought the OLM's are based upon the lowest based denominator oil. In the OP's case that's not the higher spec'd oil from GM. So using the higher spec'd oil should allow him to go safely to 0% and or longer than the OLM indicates
. Also the greater the oil sump capacity the longer the OCI can be. I don't know whether the OLM takes this into consideration.
Whimsey
Whimsey