Own 05 Civic. 140k+ driven. Still using Caltex Fully Synthetic and it does not reduce at all. Even though i have a leaky sump and find a drop or two overnight but its not enough to drop level on dipstick within 6,000k.
Engines that do start consuming oil is almost always because of worn out rings. You don't always see the smoke while the oil is being consumed. My old 2002 city used to consume about a litre in 5k of Havoline formula 10W40. The engine lasted 200k (until i sold the car).
IMO never switch to a lower grade oil after 60-80k kms. The guy at dealership will always ask you to switch since your car is "old" . Don't do it. Stick to the fully synthetic.