As far as I know the onboard computer does not measure the quality of the oil at all, it simply calculates the next change based on variables such as driving style, miles driven, etc. Therefore, doing a mid-cycle change (which most people on this board do) will not increase the service interval. When I do a mid-cycle oil change, the maintenance countdown is always exactly where it left off.
Additionally, the CBS data will not display a spark plug change. It actually specifically says this in the supplemental manual:
"Spark Plug Service (not shown in CBS): Replace at 37,000 miles."