Eco mode not the most economical.
 

Eco mode and oil changes.
Hyundai oil change intervals for the hybrid sonata is 7,000 miles and uses 5w-20 oil.
If I run eco mode all the time, especially in winter the ecu constantly shuts off the ICE and never gets up to full temperature. That cuts my oil change I interval to 3,000 to 3,600 miles. The oil get loaded down with fuel, thins the oil wears the engine faster, also increases possibly of slug build up running cool, fuel soaked and wet.

For summer I have the problem sorted, just driving in eco mode I was getting about 5,000 miles on an oil change. Now I get full 6,000 to 7,000 mile oil changes no problem. My solution is run it in sport mode occasionally on the highway, only loses about 2mpg in the summer. Goes from 40mpg down to 38mpg. Then when I get to town the battery is fully charged and put it in eco mode and that pretty much makes up for the 2mpg I lost on the highway.
I still see some fuel going into the oil. So my solution is fill the oil up just past the low mark when I do an oil change, run that oil for around 3,000 miles, then fill it the rest of the way with 20w-50 oil that keeps the viscosity where it needs to be through 7,000 miles even with 1 to 2% fuel in oil.

Winter is much more of a problem. Initially I was just changing the oil when I could strongly smell fuel, about 3%, at 3,000 to 3,600 miles. I tried the just past low oil change, then fill to full at 2,000 with 10w-30 oil, it wasn't thic enough to make a difference.
Between running sport mode as much as possible, it's my wife's car, so when I drive it and adding 20w-50 oil around 2,500 to 3,000 miles.
Now I get 5,000 to 7,000 on the winter oil change, depends on how much highway driving and I change it when I smell strong fuel smell.
I see up to 4 less mpg in sport mode in the winter driving in town and 2 less on the highway.

How do I know what the viscosity is and how much fuel is in the oil?
Testing.

Add insulation to the engine in winter?

Yeah, I'd recommend grill block at minimum, and consider oil/coolant heater for winter.

Surprised to see eco/sport make any difference at all. Many cars it just makes the accelerator mapping twitchy and perhaps insignificantly save some energy in AC use.

Are you using full synthetic oil? In my Hyundai Ioniq hybrid I only change the oil once ever 10-12k miles and never have any problems and I’m at 115,000 miles on it now

Oil base is irrelevant to the contamination of the oil with fuel.

In this car eco mode kills the engine when braking, keeps it off while the car is stopped unless the battrry is very low and only might fire the engine when accelerating from a stop, depending on state of charge and rate of acceleration.
Sport mode almost always fires the engine when the brake is released, doesn't always kill the engine when slowing down and if the battery is low enough, below 2/3 charge it will run the engine while stopped to top off the battery.

yes i know. i have the same power train with my hyundai hybrid too. i run sport mode if i want my ev battery higher, or ill run sport mode in the winter because its better at providing heat. these hybrids suck for warmth doing city driving in the cold winter!

In winter time if I ran in eco mode all the time the car would burn around 90 gallons over the distance of an oil change. If I ran it in sport mode all the time which I don't I would burn about 10 gallons more. It gets ran maybe half the time in sport mode, at most, so let's say that uses 5 additional gallons.
So around 5 gallons of gas burned nearly doubles my winter oil change distance. An oil change takes an hour and costs nearly $50 using penzoil platinum, the oil thats created from natural gas and a wix oil filter.
Seems woth it to me.

In summer I occasionally put it in sport mode on the highway and when I get to town switch it over to eco mode and run on battery power pretty much the entire time to conclude business. Very minimal impact on fuel economy over all.
That keeps the fuel in oil down to around 1% and gets me to the full 7,000 miles no problem. If I run it in eco mode all the time the summer time oil change gets cut to 5,000 or 6,000 miles.

I guess eco mode is just a mode to choose and not the stock setting because you really need to fire the engine most of the time.

Are you even testing your oil quality or are you just going off owners manuals and random made up math numbers? I’ve had my synthetic oil tested after even 12,000 miles of winter eco mode and it’s still looks barely used. I even asked the technicians their opinion on my “ dirty oil” when changing it and they’d say it looks like I’ve only driven 4,000 miles yet I’d be clocking In 10-12k miles. They’d say the oil looks like it could go another few thousand miles but I’m the paying customer so it’s up to me if I wanna continue to change it. Again, I’m doing Valvoline full synthetic oil changes every 10k miles, sometimes more, sometimes less, and I don’t stress about it. I always run in eco mode. Sport mode only in mornings to get hot air in winter, or sport mode in city driving when I need to boost it fast or interstate merging in heavy traffic

Before you accuse some one of making up numbers, maybe you should read the whole post.

How else would I know the numeric percentage of fuel and that the viscosity of the oil is out of spec because of fuel dilution?
Or that adding 10w-30 oil doesn't thicken it up enough?