[modern] Cost Benefit of running Synthetic Oils

I am the Maserati Biturbo owner Biggy was talking about. My 84 Maser
gets the Mobil 1 5w-50 and my 96 VDP gets the 20w-50 Castrol. The
Maser is down for new valves during which I removed the pan to clean the
sump. The oil was nice and clear but the case and sump was lined with
powdered coked oil. I guess the PO did not use synthetic. The Jag gets
new oil every 3000 to 4000 and runs cold compared to the Maser so no
benefit from synthetic. I also have experience with a 65 Corvair
Spyder, single factory uncooled turbo. Synthetic was not available at
the time and recommended max. change interval was 3000. I remember the
oil was looked bad after 3000 miles and I never could reconcile this
with the 62 Porsche 356 and 64 Jag Mark II I had at the same time.
Their oil came out so clean at 4000 mile I considered putting it in the
Corvair. I don’t think I ever did reuse the oil in the Corvair but I
sure thought about it and I did use it in lawnmowers.
For the time being I’ve got all I can handle to get the Maser back on
the road and probably could not keep up with the longer change interval
if I changed to Synthetic in the Jag. and my wife Mini van that gets
changed the same weekend as the Jag.