Be easier if there was an oil temperature gage.
Can use infrared temperature gun on the oil pan for a ballpark (assuming its not a shiny or chrome pan)
The coolant temperature otherwise is next closest reference.

Trial one multi grade down to see how it effects both the idle and the higher revs.
That would be a multi-grade 40. See what is available in 10W or 15W-40.

Or. if you prefer sticking with the current product line, you can mix in a percentage of lighter grade.
Last I checked, this calculator still worked.
https://www.widman.biz/English/Calculators/Mixtures.html

Get the viscosity for the oils in question from the manufacturer.
Sometimes its on their websites. Others make you ask for it.

Then watch the temperature and pressure at idle and high rpm.
If the pressure drops at idle, but stays the same or is higher at high rpm, then that's better for the engine.

I like synthetic. Used Amsoil. Filter looked better. Parts looked better. I used 10w30 and the clearances were bigger than they should have been. Oil pressure would have made most people pucker.

let me explain further.

if at 3000 rpm, the pressure is 80 psi with a 50
and after filling with a 40 wt, the pressure is still 80 psi
then the same or more oil is flowing through the engine.

Why more?
Imagine trying to push molasses through a system. The system could be bottle filling system. Whatever you want to imagine.
Then picture using the same effort (pressure) to push honey or maple syrup through the system.
That same pressure is going to flow more maple syrup and flow it further.

Keeping sufficient oil to float the bearings etc requires both pressure and flow (psi and gal/min)
The faster the engine spins the more volume per time is needed at the bearings.
So the goal is to have the rising pressure increase the flow as far as possible up the rpms.

Ok I agree but this is the way it was explained to me I aviation engine school decades ago:

The higher the viscosity the longer it takes for it to return to the oil pan. A high volume pump can suck the pan dry in a racing situation as the oil is draining back through the valley pan area into the pan. Which is why dry sump systems exist as well as large volume oil pans. Dry sumps don't rely on gravity to return the oil to the pan. Large oil pans hold so much oil they can't run dry. And, I'm guessing this car will never see below 32 degree weather so a multi viscosity oil really doesn't matter. I agree with the oiling recommendations listed above though. If it were my engine I'd run something with a 30 or 40 weight as the second number. I prefer full synthetic. Its a hell of a lot cheaper than a engine rebuild. Oil is one if the things it doesn't pay to be cheap on. Like brake pads...think about it.

Just my pennies,
CC

You can’t make that statement if you don’t know what the clearances of the engine are. It’s not a stock engine.

I made that statement based on his hot idle 55 psi with 15w-50. That’s far more pressure than needed, regardless of bearing clearances. Period.

He has room for improvement by either running thinner oil, or less volume (different pump). Since he’s currently running fairly heavy oil, trying a lighter oil at this point makes perfect sense...as others have suggested.

Yes, on the thinner oil and see what you get.

I have been told by a very experienced machinist that synthetics don't cling to parts long term so on a hobby type car that sits a lot you may end up with cam lobes that have less oil film on them. True or not I don't know for sure...… This guy has a 7 second drag car and he uses conventional oil in it.

That same comment is used to describe high detergent diesel oils.....(Delo, Rotella etc). The detergent greatly affects surface tension of the oil film and causes it to collect together and therefore drain from vertical surfaces faster. On cam lobs it drips off faster on shut down rather than clings as a film. Dryer starts are the result. Don’t know how big of a deal it really is, but apparently that’s one disadvantage of detergent, the other being a possible negative reaction with ZDDP, but HDEO’s are formulated that way, and they certainly work.

Makes sense.

The other night I noticed that Widman has updated his Oil for Corvairs paper and now is a little more generally about flat tappets and includes more on High Temperature, High Shear viscosity (HTHS). Limited comments about specific oils but Dr Charles should look at page 24.

He has also added a paper on oil for Minis (real minis) which is worth glancing at just for its introduction to gear oiling.
https://www.widman.biz/English/Bulletins/flat.html

Over on a motorcycle forum we are in a big long debate on using Shell Rotella T6 5W40 that is Jaso MA2 rated for wet clutch use. The main idea is that it is not what the Suzuki engineers qouted in the service manual. The manual lists 5W30, 10W40, 10W50 but not 5W40 and a (Anal Retentive) is saying we are stupid for using 5W40.

But my point of this is we have two British mechanical engineers members that have done testing of 'Car' oils in motorcycle engines that rev to 14,000 RPM. They say that car oils do not have the 'CLING' that true motorcycle oils do and at 14,000 RPM almost all oil is slung off cams and such.
It does not matter that 1,000's of motorcycle riders have used Jaso MA2 rated Delo 400 and Rotella T6 with no major failures. The Suzuki engineers are gods and they did not say it was OK. ??? 5W30 vs 5W40???

My son worked for Rob Muzzi for 23+ Yrs, Rob was the Kawasaki factory road race team owner and manager for many, many years, they also sponsored his NHRA Pro Stock bike team for several years. They where revving those 750 and 1100 CC road race motors over 14,500 back then
My son introduce me to his head mechanic back in the mid 1990s, they where using Motul synthetic racing oils only in their bikes back then, I use to see it in the race shop until Rob retired
I don't like synthetic oils for my own reasons based on results from dyno testing and the synthetic oils leaking, seeping, weeping, through many American made gaskets for our old Mopar motors, pure oil based oils don't leak through them
Mobil Oil Corp was sued by the Contentinal (SP) Aircraft Engine owners assoc. on their Mobil AV 1 synthetic aircraft oil because it wouldn't stick on the camshaft lobes and lifters when those motor would sit for more than a week without running. They won the law suit( a bunch of money, a BUNCH) and Mobil quit making and selling that oil right after the law suit was settled
I had used that oil for over 7 yrs in my Piper Comanche 250 and it would not hold gunk in the oil, it would leave deposits all over my airplane motor where it would sit and puddle, I had used Shell aircraft oil before that and that motor had no gunk in it anywhere when I bought it, I had the plane inspected thoroughly before buying it due to it had sat for over 18 months without flying, I was able to remove a lot of the inspection panels and all the rocker ram covers to look inside that top end, we did a leak down test and cleaned and gapped all 12 spark plugs. It turned out to be a very good air plane
I flew it for 15Yrs and a little over 900 hours before it got totaled on the ground by a hail storm here in Bend, OR d
I think all the new cars and H.D. trucks come with synthetic oil in them from the factory now

That same kind of mentality is evident on the Hellcat forums. Now I understand that complying with warranty requirements, no matter how asinine, is imperative in order to keep the warranty valid. But other than that, most opinions are total nonsense. Not based on any real experience.

For most of our cars, there is wide range of oils that will serve our engines well. 5 vs 10 weight, or 6 months change vs 12 months change, and yes, syn vs part syn vs petroleum are all a bit ridiculous to argue over. Heck, even the petroleum oils are largely synthetic. No one believes they are straight crude out of the ground.

I have asked a number of times, in a number of venues; how much of a quart of VR1 is petroleum and how much is synthetic additives? And what percentage of synthetic additives does it take to be able to be called semi-synthetic? Does anyone know the answers? I have never been able to find out.

I run Valvoline VR1 in my Hemi, change it once a year and I beat the snot out of it. No oil related failures. How much better than that can you get?

Those original synthetics were ester and PAO only oils.
The stuff sold today as synthetics are different in two ways. You shouldn't have those problems with better 'synthetics' today. That's not to say you ought to try them. Bad experiences like that are a powerful incentive!

You may be able to figure it out what the base stock(s) are from the material data sheet of labelling.
When you read Widman's paper you'll see that's where he gets the info, although sometimes he has to figured out based on clues in the documents.

here's a some basic overview of the base stock snipped from https://www.machinerylubrication.com/Read/29113/base-oil-groups

API Base Oil Groups
In the early 1990s, the American Petroleum Institute implemented a system for describing various base oil types. The result was the development and introduction of base oil group numbers.

Group I base oils are the traditional older base oils created by a solvent-refining technology used to remove the weaker chemical structures or bad actors (ring structures, structures with double bonds) from the crude oil. Solvent refining was the primary technology used in refineries built between 1940 and 1980.
Group I base oils typically range from amber to golden brown in color due to the sulphur, nitrogen and ring structures remaining in the oil.

Group II base oils are created by using a hydrotreating process to replace the traditional solvent-refining process. Hydrogen gas is used to remove undesirable components from the crude oil. This results in a clear and colorless base oil with very few sulphur, nitrogen or ring structures.
In recent years, the price has become very similar to Group I base oils. Group II base oils are still considered to be mineral oils. They are commonly used in automotive engine oil formulations.

Group III base oils are again created by using a hydrogen gas process to clean up the crude oil, but this time the process is more severe and is operated at higher temperatures and pressures than used for Group II base oils. The resulting base oil is clear and colorless. [I]t is more resistant to oxidation than Group I oils.

The cost of Group III base oils is higher than Group I and II. Group III base oils are considered mineral oils by many technical people because they are derived directly from the refining of crude oil. However, they are considered synthetic base oils by other people for marketing purposes due to the belief that the harsher hydrogen process has created new chemical oil structures that were not present before the process. It has synthesized (created) these new hydrocarbon structures.
Group I, II and III base oils basically reflect the evolution in refining technology over the past 70 or 80 years.

Group IV base oils are polyalphaolefin (PAO) synthetic base oils that have existed for more than 50 years. They are pure chemicals created in a chemical plant as opposed to being created by distillation and refining of crude oil (as the previous groups were).

PAOs fall into the category of synthetic hydrocarbons (SHCs)...and are significantly more expensive than Group III base oils due to the high degree of processing needed to manufacture them.

Group V base oils comprise all base oils not included in Groups I, II, III or IV. Therefore, naphthenic base oils, various synthetic esters, polyalkylene glycols (PAGs), phosphate esters and others fall into this group.

Very interesting read, thank you. Also the top of page 29
Back to VR1 10W-30!

And you can mix 20W-50 in the same if it looks like that's needed.
it would get something like a 15W-40 so not such a big experiment along with its higher HTHS

Do not use the pressure relief to adjust or set your pressure. It's an upper limit, consider it a bypass. If you are constantly on the bypass, you are forcing too much oil into the bearing clearances.

The only engines that should be using 20w50 are worn out engines that need oil pressure because you do not feel like fixing it properly, like a rebuild.

Same with High Volume oil pumps. They were made for installing on a worn out engine.

Anything with clearances within spec will need a standard oil pump, 10w30 and you will have enough oil pressure for any conditions. I do totally agree with DaveJ's comment on horsepower. We build these engines with performance in mind. So why send more power to the oil pump drive than you have too?

I disagree on the use of the high volume oil pumps, they where designed for racing motors, not worn out stock motors
I like and use loose bearing clearances on all the motors I build, race or HP street motors. I've never seen or heard of a rod bearing spinning do to much (.002 to .0035) oil clearances, same thing on main bearings (.0030 to .0045)
I like and use the 10 lbs. per thousand RPM rule with hot oil (150F+)and trim or shim the bypass spring to get that if needed
I also run 5W20WT oil on my street motors now, Valvoline VR1 10W30 WT in my race motors
I use to use 50W racing oil, years ago
I've done a lot of engine dyno testing and found out that the oil temps can have a direct affect on HP, 30 W oil at 130F would not have as much power as it would at 170+ F oil temps, 8 HP gain with hotter oil I then tried 10W30 wt and saw 5 HP differences between 130 F and 160F, the next test was with straight 20W and I saw no HP gain from 70F up to 210F
That is why I use that weight oil now in all my street motors and low (below 750HP) HP bracket motors

I have watched so many Dyno runs on TV. They always do major changes between pulls so you never know what was the major gain in power.

But we did a Dyno run on a 513 BBM years ago using 20W50 racing oil. All tests were at 180 degrees F. We then replaced the HV oil pump with a standard one and gained 10 HP. Then we dumped the 20W50 and ran 5W30 and gained 15 Horsepower. So a gain of 25 Horses. After 30+ runs the main bearings were pulled and had very minor wear as was normal.

I have not run anything above 10W30 in any car except my Subaru Outback XT turbo, it gets Rotella T6 5W40.

Do you remember how much oil pressure it had with both pumps above 4500 RPM? Is that information on your dyno sheets?
I have had to trim a lot o the black high pressure bypass springs to get to have 65 lbs. at 6500 RPM with 5w20 wt. oil in my street motors at or above 130F oil temps.
I think I saw a small HP and torque gain by cutting the pressure back on a high volume oil pump on one of the dyno tests done years ago from 80 lbs. at 5000+ RPM down to 65 lbs. at 6500 RPM with the oil temps above 150 F, maybe 3 HP, maybe less
I've never had a bearing problem with 10 lbs. per 1000 RPM rule on any motor with loose bearing clearances, I have seen rod bearings spin in other motors at the drag races that had less than .0020 rod bearing clearances on BB Mopar motors when raced with cool oil temps

Is there a 15W-40 that still has 1100-1200 ppm zinc?

When I broke in the cam I used 10W-30 with a bottle of ZDDP additive. But when I dropped the pan to fix an oil leak, I found black additive slime in various places.
Now I only want to use an oil with the proper amount already in the formula.

For those who say never use 20W-50 in any engine, that depends on the manufacturer.
BMW's M50 four-valve inline six specifically calls for it in any temp above 14F. They made hundreds of thousands of them between 1990 and 1996. (I have a '93).
A common problem is owners not doing their reading and putting 10W-30 in them. And the tappets start clattering, not the bearings. Put the proper weight oil in and they stop making noise. When I got mine it had a quickie-lube sticker showing 10W-30, and very noisy lifters. I changed it for the correct oil and voila... all quiet again It's still on the original engine at 327k miles, too!