I own a 2015 Honda Mobilio 1,5 that I bought new in February 2015 after reading your road test (February 2015). I check the oil, water and brake fluid levels regularly. At the first check, which was at 1 000 km, the oil level was at the upper mark of the dipstick. The oil level increased over a 12-month period, which Honda could not explain.
I took the vehicle for the first service (only 4 600 km on the odo) and the oil level at the time was 5 mm above the full mark on the dipstick. Honda replaced the oil (level up to the full mark of the dipstick) and told me to bring it back after 1 000 km for inspection. Even after 300 km, the oil level again increased to 4 mm above the upper full mark of the dipstick. The dealership drained out 0,5 litres and told me to monitor the situation. My logbook shows that the petrol consumption deteriorated from 7,7 L/100 km to about 8,3 L/100 km in the past 12 months.
If petrol is the reason for this problem, my question is: how much damage is already being done to the engine and if not petrol, what can it be?
The best way to determine which substance is causing the oil level to rise is to get a sample tested. There is a small chance that it’s coolant, but the most common problem is oil dilution where fuel contaminates the oil. This also ties up with your fuel consumption measurements.
Fuel reaches the oil in a petrol engine when the engine runs extremely rich (remember, the stoichiometric ratio for petrol combustion is 14:1 air-to-fuel). Some of the unburned fuel gets into contact with the cylinder bore walls and washes past the rings into the sump.
It is easy to test the air-fuel ratio by using a gas analyser in the exhaust. We expect that a sensor (like the mass-airflow sensor or lambda) is to blame, so conduct a diagnostic check, too.
The danger of oil dilution is that fuel washes the thin film of oil on the cylinder bore walls away and this increases the wear rate. Depending on the percentage of fuel in the oil, the lubricity of the oil also deteriorates and leads to wear elsewhere. As the vehicle is still under warranty, Honda should urgently fix the problem.