WHAT?! They put a sticker on a windshield suggesting an oil change
interval of 15,000 miles?! That is stupid and I don't care what kind of
oil it is. Generally speaking, under normal driving conditions, you can
expect dino oil to go 5,000, maybe a little longer.
Semi-synthetics start to deplete their pack (TBN) around 7500 miles and
although some synthetics say they can go higher, I have seen used oil
analysis reports suggesting synthetics should be changed at no more than
10,000 miles. With severe driving conditions (heavy towing, cold
start/short trip driving, dusty conditions, etc.), these figures would
be much less.
Going 15,000 miles on this oil is a recipe for disaster. I don't doubt
it can be done since you will rejuvenate the oil by several top-offs
during this period but at what cost to the engine? If it were me, I
would change this oil after 5,000 miles, send in for an oil analysis
report (about $40) to Amsoil, Blackstone or some other well established
lab, and look at the results. I bet you will never see a professional
lab analysis recommending this oil to 15,000 mile intervals.