The 'viscosity' of the oil will have a very small effect on mileage, however unlikely that can be measured with any accuracy. In addition the 'blend' would even have a smaller impact if any.
For example 5W20 motor oils were introduced into North America to meet CAFE emissions targets across several thousand vehicles, with any one vehicle having only a very small impact.
Regarding the sensor (copied from another post on the forum...)
The manual indicates the following:
Your vehicle is equipped with an automatic oil change indicator system. The oil change indicator system will remind you that it is time to take your vehicle in for scheduled maintenance. Based on engine operation conditions, the oil change indicator message will illuminate.
This means that service is required for your vehicle. Operating conditions such as frequent short-trips, trailer tow, extremely hot or cold ambient temperatures will influence when the “Oil Change Required” message is displayed. Severe Operating Conditions can cause the change oil message to illuminate as early as 5,600 km (3,500 miles) since last reset. Have your vehicle serviced as soon as possible, within the next 800 km (500 miles).
Your authorized dealer will reset the oil change indicator message after completing the scheduled oil change. If a scheduled oil change is performed by someone other than your authorized dealer, the message can be reset by referring to the steps described under “Electronic Vehicle Information Center (EVIC)” or “Driver Information Display (DID)” in “Understanding Your Instrument Panel” for further information.
NOTE: Under no circumstances should oil change intervals exceed 16,000 km (10,000 miles) or twelve months, whichever comes first.
Severe Duty All Models: Change Engine Oil at 6,500 km (4000 miles) if the vehicle is operated in a dusty and off road environment. This type of vehicle use is considered Severe Duty.
Additional information if it helps...
The Oil Change Indicator system is not a "sensor" that interacts with the oil. In fact if you changed out your oil and put in water or apple juice, you would have serious issues, however it would not indicate you needed an oil change !
The Oil Change Indicator system uses the Powertrain Control Module (PCM) to process a variety of inputs and then decides on an oil change interval. The factors which are measured include:
•Ambient Temperature (looking for low temperature operation)
•Average Vehicle Speed (indicate stop-and-go driving)
•Run Time / Prolonged Idling
•Trip lengths / Coolant Temperatures (indicate shorter trips)
•Engine RPM / Engine Load (indicate trailer tow usage)
•Fuel Indications / Ethanol Percentage
Based on those items the PCM can determine the type of use and then appropriately send out a message to change the oil in advance of the 16,000 km, sometimes well in advance. It is far from perfect, however better than someone not measuring anything and not being alerted at all to that they 'may' require an oil change service prior to the 16,000km / 12 month benchmark.
Similar to what has been mentioned by sciond I also strongly recommend the
www.bobistheoilguy.com website. The type of oil can play a role. 5W20 by its very nature is at least a blended synthetic in all cases, however not all motor oil is created equally. What I have seen when it comes to used oil analysis (UOAs), brand names such as Mobil 1, Pennzoil Platinum, and Amsoil are generally being mentioned more frequently than others allowing for a bit more buffer should you forget or draw out an oil change.