Sadly, nobody is right all of the time. I'm afraid it's your turn, spindleshanks.
Dino oil will expand oil seals and gaskets, while synthetic oil will not. I has nothing to do with viscosity. I speak from personal experience with my own leaky vehicles, in addition to my profession in the automotive industry.
If your oil is "clean" - low particulate count, while maintaining it's viscosity, changing the filter only is a perfectly viable maintenance method. There are bypass filters on big-rig semi trucks that use bypass filters with entire paper-towel rolls as the filtering element, extending the oil change interval significantly.
A little research goes a long way...
