Oil
I tend to agree, but, if his oil pressure isn’t spectacular with the 20-50 going to 10-30 might not be ideal.
I need to get a pressure gauge installed. It’s all original with original Rally Dash. All gauges operational. Oil pressure gauge slightly over 1/2 way point on start up and 1/2 way consistently. I’m a purist and didn’t want to add gauges to the interior. I may tuck a small one under the dash.
OK. I'm going to try to connect the dots here for you.
Oil viscosity changes dramaticly with temperature.
The thicker the oil, the more difficult it is to pump and push through the galleries.
When the oil is cold, it is so thick that even at idle a bunch of it goes out the relief valve on the pump instead of through the engine.
When the oil gets up to operating temperature it needs to be thin enough that plenty flows through all the way to the top. It also needs to be in enough quantity and at high enough pressure to float all the bearings. In other words the metal parts should not touch but be supported by the oil flowing between.
So....
The oil pressure gage should show relatively high when starting. The actual temperature makes a difference.
The oil pressure on the gage at idle should go down as the oil warms up.
The oil pressure should be around 10-15 psi at idle and go up with increasing rpms. Old rule of thumb is 10 psi for every 1000 rpm.
It likely will max out at some rpm. That's normal. Might be 4000 rpm. Could be lower, could be higher.
That said, running too thick of an oil will cause the oil pressure to max out at too low of an rpm, and could result at oil starvation at much higher rpms.
Observe the temperature gage, the weather conditions, and the pressure gage. Then you can decide.
Unless it was intended as a race engine, or is really worn, its probably not that loose.
I'd start with a 10W-30 or 10W-40 and go from there.
With a purpleshaft it likely has higher pressure valve springs, so an oil using anti-wear package with decent amount of Zddp is a good idea.
However I would not fret over it either.
Ford Superduty 10W-30 is a relatively inexpensive and readily available choice.
Brad Penn has a wider selection of choices but is less readily available.
Some of the racing oils have less detergents and so forth, so check with the manufacture's info whether they are suitable for street use. Obviously you can ignore any concerns about catalytic converters.
