[Synchro909]

I have explained here before that when I am touring outback with a camper trailer in tow, I judge when to change the oil by watching my instruments.
The engine has oil and air filters, 5.5:1 head, a downdraught Stromberg carburettor from a 1950's Holden and a very mild cam. The middle main bearing is pressure fed at 25 psi. With new oil and travelling at 50 mph and 180 F temp, the oil pressure is 25 psi. Eventually, at the same speed and temperature, the oil pressure starts to drop. The oil has "lost its guts". When the pressure is down to 23 psi, I change the oil and that is quite reliably at 3,500 mile intervals. I use the cheapest 20W50 oil available and ignore the colour of it.
Just my experience. BTW, I have done about 40,000 miles on that engine like that so far.