While I am quick to condemn dealers, one has to wonder what happened to COMMON SENSE. Does no one read the effing manual anymore? All it takes is a little research in the glove box and if not, ten seconds on the internet, and you will find out things like the change interval for spark plugs.
If you want to find out how stupid and gullible people are today, simply walk down any street and count the vehicles with the lime green valve caps. ALL those people have been scammed. EVERY SINGLE ONE OF THEM. There is zero scientific basis for filling tires with nitrogen, and dealers (yes, and even your local garage) use them to increase their profit margins.
I have heard all the junk science.
1) "Nitrogen molecules are bigger and leak less, especially in cold weather." This works for people who slept through science class. Do people not know the size of a molecule? There are ten million in an area the size of the period at the end of this sentence. Nitrogen and oxygen are side by side on the periodic table (which is numbered by SIZE) and therefore there are about ten million and one oxygen molecules in an area of that period. This is bullshit science.
2) "Race car drivers use nitrogen because it is more stable." No we don't. We used nitrogen in our race car because it was free, and came in fresh green tanks instead of the air that we neglected to drain the moisture out of for about 200 days in a row.
3) "Airlines fill their airplane tires with nitrogen so it HAS to be better." True. They do this because it expands less at temperature extremes and will have less tendency to feed fires in a crash. But unless your car tires instantly go from minus 60 at altitude to plus 300 and then have to tuck into a tight wheelwell with inches to spare on a balked landing, you have nothing to worry about.
4) "Maybe it doesn't do anything, but it was free with the car (or the tires.)" Hahahaha. You REALLY think it was free? Nothing is free in life, and if you don't believe me, read the fine print in that free trip to Vegas your dealership gave you for buying your car. Free? Hahahaha.
Now let's contrast those with the FACTS:
1) Nitrogen is, at best, 98%. And that is assuming the garage has maintained their tanks, cleans them regularly and takes the time to generate the nitrogen. Air is 78% nitrogen, so in essence, you are paying for air.
2) If nitrogen molecules leak less than oxygen molecules, wouldn't you save money by just filling your tires with air, and as the oxygen molecules all leak away first, just keep topping up your tires, and soon you will approach 98% nitrogen? The whole argument is silly, because there is no scientific basis for this.
3) "When I see those lime green valve caps on a customer's car, I know I can sell that sucker ANYTHING." True quote, from an anonymous service writer.
4) "We ran out of nitrogen years ago. We just fill the green tanks with air and no one will ever know." Anonymous former mechanic.
Of course dealerships (and your friendly local trustworthy garage) will try to upsell you. It is how they stay in business. EVERY business tries to upsell in some way or another. The sun rises in the east. So what? It's a fact of life, and always will be. Do some research and learn how to say no. It's business. It's unethical and mildly dishonest, but it's the car business. Did people HONESTLY think they could buy a car at below dealer invoice pricing? Hahahahaha.