[Kurt S]

I do shoot for 5K between changes. I consider it cheap insurance.
The basic issue has changed over the years. The VI (viscosity index) improvers that are added to the oil to make is thicker at higher temps (giving it the multi viscosity rating, eg 10W-30) used to break down. Now they've extended the life of these and other additives. Hence the longer recommended oil changes.

I don't run synthetics in the daily drivers, mainly due to cost. But have in the past, esp in manual trans'.
I will be running it in the Z, esp the trans and axle. I saw where there was a Hemi that couldn't break the 11's. Changed to all syns, that did it.