That’s a pretty fair but simplistic assessment.
Before 20/50 came out, there was just straight grades. If you look at older car manuals, it specifies them. Then 20/50 came out, but that was the best at the time. Since then things have moved on, which is why newer engines of the SAME type as an older engine, will specify different oil.
Not because the engineers messed up, but because new oils are available.
It’s common now for cars to do 300k on an engine and some to do 500k. I never used to see many of the oil cars get past 100k with 20/50.
I also suspect that just because it may have been used in the 600, I’m unlikely to come across people that have used it SUCCESSFULLY to put 100,000 miles on a motor. Any oil if changed regularly will appear to do a good job if you don’t do high miles.
The fact that the oil pressure climbs so high when cold indicates a significant restriction to flow and if the pressure relief in the oil pump is at 35psi then you’ll be bypassing through the pressure relief and thus having less oil round the engine at the critical cold start times. Using a more modern engine oil that gives similar viscosity when hot (a 40 or 50 grade) but a lower when cold (a 10 or 5 grade) will mean lower restriction when cold and thus the pressure will drop down a bit due to the better flow and ensure the oil pump bypass isn’t operating and thus the full oil flow is getting round the engine. It also can halve the time the oil takes to get up the top end, not to mention the longer chain polymers in high tech oil do a far better job of preventing wear.
There are places for 20w50. I have used it on motors that are rattling and burning a little oil, it’s thick so quietens them down a bit – the motors will be dead in a month anyhow so who cares if it hastens the demise. On very worn oil pumps they won’t actually pressurise thin oil because it leaks out of the pump, so putting some treacle oil as I call it in may just get you home or get the thing running one last time before it goes to the great scrapyard in the sky.
Greg.