re: OT mechanics how long does synthetic oil last

The oil doesn't wear out. The additive package does.

It all depends on the application.

Some engine are much harder on oils than others.

In a large v8 app, you likely have a parge sump. If you arent towing and doing highway mainly, 15k isn't that outrageous. However, if at an oil change place, it is pretty unlikely you saw anything more than mobil1 ep of whatever they are calling pennzoil ultra these days.

It's even more likely that: your sump is filled a half qt or more low, the oil they said is in there is actually just bulk oil, they wiped off your old filter really well...but didn't change it.

Personally, I'd change it out at 7500 miles. And then do an oil change myself with mobil1 0w40 and a fram ultra filter.

Then you might be able to purpsue longer OCIs.