I know that Volvo owners are a fussy lot but the qualities implied with synthetic oil are nothing short of miraculous.
Perhaps this oil change obession comes from the old days of low grade oil, gas and engines.
I follow the recommended Volvo intervals with regular oil. (10 k miles)I don't think they are careless engineers.
Look at the studies conducted in various engines. After an initial decrease in viscosity and increase in heavy metals concentration, nothing much changes out to 15k miles typically. Despite being visibilly darker, carbon levels rise very slowly. Synthetics typically have smaller viscosity drops.
I've added Marvel Mystery Oil to the crankcase prior to an oil change. It cleaned up the camshaft a bit.
Maybe I don't want to do that. Maybe the varnish is keeping the seals together.