Short distance city driving is actually quite hard on the engine and the oil. 15min is not enough time for the engine to properly warm up. As a result, accumulated gasoline and other contaminants from cold starts are diluting the oil instead of being evaporated and burned off. This is weakening the oil's lubricating properties and could damage the engine.
I would try to get some longer distance/duration driving in if you can. Possibly on a weekly basis.
Another concern is the effect such short distances will have on engine break-in. A few longer city trips (not highway) with lots of stop and go driving will help with this. This will help to seat in the piston rings and other components so you get the best 'seal' and improved fuel economy and performance.
As for the oil changes,
Without doing a used oil analysis, it hard to recommend something. With such short distance driving, duration becomes more important than mileage. Here is what I would do:
- Get a clean plastic bottle/jar that holds about 120ml (about 4oz), with a good lid (screw cap). Hint: look at food containers, etc at the grocery store.
- Do an oil change as the dealer suggests. I also recommend going for a minimum 25 minute drive, immediately before your oil change to evaporate any gasoline in the oil. This is necessary for proper oil analysis.
- As the oil change is being done, ask them to collect an oil sample using the bottle you provide. The oil should be collected as it is draining from the motor. Don't scoop it from the pan or collect as soon as it starts draining.
- Send your oil off for analysis. There are many labs, here are a few:
http://www.wearcheck.ca - Located in Burlington, Ontario, Canadahttp://
www.blackstone-labs.com - Fort Wayne, Indiana, USA
When you get the analysis back, please post the results here. Then I can recommend how frequently you should change the oil.
If you don't want to go through the trouble of oil analysis, then stick with the 3 month oil change interval. Yes, it's frequent but I am quite concerned about the short distance driving you are doing.