My driving mileage varies different time during the year. For the most part I drive more during the summer. Whenever I get an oil change the dealer will stick a sticker on the upper left corner of my windshield of the recommended mileage and date for my next oil change. Which should I follow more, the date or the mileage? I tend to just follow the mileage.
In a sense it doesn’t really matter. As long as you change your oil regularly, by whatever marker you choose, your car will live long and prosper. Keep doing whatever you are doing. It works for you.
First the car manufacturer specifies when the oil (and many other services) should be completed. You should have them done on the manufacturer’s schedule.
If it gives both a time and mileage, it means which ever one comes first. So if today you get the oil changed and it says one year or 6,000 miles then when you drive 6,000 miles without a change or if a year has passed without a change, it should be done.
Notice I said that you should follow what the manufacturer specifies (see owner’s manual). The dealer may be using the manufacturer’s recommendations, but some make up their own and you can bet they will have you change oil more often because they like the business.
A second issue is having the dealer do it. There is no need to have the dealer do the work. Some dealers do a fine job and don’t try to sell you something you don’t need and a few (very few) dealers don’t charge more than the going price. A good independent mechanic is as good as a good dealer and will usually cost you less. It’s your choice.
Finally don’t switch to using any quick lube place. The are the worse option of all.
The best option might be having your son do it.