15,000 miles converts to 24,000 kilometres. Personally I prefer to follow the manufacture guidelines, as I find dealers can have exterior motives, increase sales and services. The manager would need to provide technical reasons for me.
I would have question the manager and asked why the change. If he felt that water was an issue, for example, then they should be draining the filter every time you bring in for oil change. Unfortunately, I suspect most don't even do that, forcing the owner to crawl underneath, probably spill diesel on themselves or paved driveway (it illegal in some municipalities) or hoping you come in, so that they can charge you for the service and sell you a new filter, just because there was water in it.
Have you notice, how many trucks are being sold with no mud flaps. Just recently, a dealer was visited by the RCMP, advising them, it is illegal for them to sell without mud flaps. Guess what, the dealer installed mud flaps on all their new trucks in their stock. That must have hurt their parts sales. I don't trust dealers in general, they have the diesel gate mentality. They are quick to exaggerate, just to make more money. Notice how they won't put nothing in writing and if any, it very vague and incomplete. We need to question them, always.
Water can be an issue, older stations may have more water than normal in tanks, not doing proper flushing maintenance. Extreme weather changes can add to the issue, I suspect. During winter, it is my understanding that they use anti-gel, keep from freezing in service stations. It is my hope, and testing, that with Shell V-power which also uses anti-gel but have additional additives to disperse the water.
On my third tank of it, and big difference, I have noticed as mention on this site in several areas.
Time will tell, if the claims are true. So far, all is well and the claims seem to be accurate.
It would be nice to have a detailed dealer report card on this site, so the successes and failure, poor service can be documented by the members.
FYI: My third oil change is coming up to 25,000 kilometres. First one was at 5,000km, then the second 15,000 km. Those changes were dealer stickies indicators.
First one was free, I pay the next two (240.00 each time) and the third one is free, etc.
When dealer/FCA advertises these specs, then after you buy it, they change it, it false advertisement or misleading. Don't you think?