I hope personal viewpoints are not seen as absolute!
Ted - you said;
"In the same engine under the same conditions, a significantly thinner oil will reduce oil pressure..."
I would add however, that a low viscosity lubricant will also typically reduce average oil temps. The difference in dropping down one SAE grade is normally a 5F-10F reduction in equilibrium oil temps. So the difference in oil pressure between say a 5w-30 and a 5w-40 is not as great as the difference in HT/HS viscosity - which is measured at the same 150C/302F temp.
The temperature "equivalency" between each successive SAE grade in terms of oil thickness is about 20F/10C. For example, the following "mid-grade" SAE samples would yield approx the same oil pressure:
xw-20; running @ 190F
xw-30; running @ 210F
xw-40; running @ 230F
xw-50; running @ 250F
All things being equal, the thinner oil running at the lower temp is preferred. Significantly lower fluid temps will reduce wear and also extend the life of elastomeric seals/gaskets...."
Ted these broad statements appear simply theoretical, are they? - sadly, they may not be too accurate!
An engine measured to the same criteria at the Manufacturer's prescribed datum point of say 4000rpm will in all probability show the same OP, be the oil a 0w-20 or a 15w-50! Because of the pumping losses alluded to by Bruce very few if any engine makers will provide a test number for OP at idle! The engine's condition - age, wear etc, may have some effect too!
I have done a lot of oil temperature testing in live water cooled engines Ted and I have not experienced a significantly meaningful reduction in residual oil temperatures in the oil pan when using less viscous lubricants! Aircooled engines are a different story!
Certain engine components may run cooler due to more linear oil flow characteristics if the change in viscosity is also from a mineral to a synthetic lubricant! Measuring the change would however require sophisticated test equipment
Generally, other factors have a greater influence on residual oil temperature than viscosity does!
Ted - you said:
"So the difference in oil pressure between say a 5w-30 and a 5w-40 is not as great as the difference in HT/HS viscosity - which is measured at the same 150C/302F temp."
Of course not Ted although M1 0w-40 and GC 0w-30 at 100C are near, and that is the point - and this is why the ACEA's HTHS Quality rating system is so valuable
This was the fallacious comment made to Buster by "the Amsoil guy" that prompted my first post;
"....the film thickenss isn't much different between a 20wt and a 30wt oil so you won't see much of a difference."
It referred to "thickness" not OP between an SAE20 and an SAE30 lubricant!
Ted I have trouble understanding this................;
"For an engine of a given design - including oil pump output, oil temps and tolerances - oil pressure IS a direct function of viscosity, or more specifically high temp, high shear rate viscosity."
Can you explain it please? - I'm have some trouble.....?
Ted this comment is meaningful;
"Changes can also be made to the piston/ring assembly to ensure proper lubrication and better cooling. For example, some engine now use "oil jets" to help cool the pistons."
In 1960 Mercedes used at least four compression ring designs and three oil ring designs that used any of four wall contact springs. Oil jets (piston squirters) for under crown piston cooling have been around for several decades too Ted.
Many German engine makers tried oil jets in production petrol engines two decades or so ago and many discarded them quickly too. Porsche used them in MY87 on some engines but only for a small production run. Oil foaming and etc is sometimes their product! Engines that continually operate at high revs have more difficulty with oil jets than those operating at low revs
(Oil jets have been successfully used in heavy diesel engines for several decades and with somewhat lessor lubricant issues. Many of these engine have flight revs at 2200rpm and operate at around 1600rpm with 1800rpm maximum)
Ring pack position on the piston and ring design have been changed in recent times to meet emission demands. Many now run very much closer to the "fire" area than a decade ago and this has introduced many lubricant issues. The average time that oil spends in the ring pack area is directly influenced by viscosity. There is some clinical evidence that lighter oils spend less time there and lubricate better than heavier viscosities (say 0w-20 against 15w-40)
In item 1) you said:
"increasing oil pump output (increases oil pressure for a given oil viscosity)"
Ted increasing the oil pump's output will only increase the volume! It will have no effect on pressure!
In you item 3) on valve trains you did not mention lighter valve spring pressures which have significantly reduced cam wear - especially with radical cams. You probably simply forgot! Many engine designs now use up to three lobes to actuate one valve!
Bruce381 - Pressure versus Flow! - This issue was discussed very well some years ago on here and Bob's comments, reports and diagrams are accurate. Flow and pressure are often confused. Pressure is as you know a product of the resistance to flow. As you know the more linear flow of (certain) synthetics (no name mentioned or I shall be judged as biased) can sometimes manifest itself as a lower OP especially at very low revs - usually under 1000rpm.This is of little consequence unless it is at/near zero!
AEHaas - you said;
"Oil pressure is insignificant in automotive lubrication. The separation (of parts) pressure of oil in a bearing is several thousand pounds verses the feed pressure of tens of pounds. Flow is however important, regardless of the pressure.
I beg to differ and this convoluted comment is conceptually wrong! How will you get flow without pressure? IMHO if you instead had said: "Flow is however important to journal bearings, almost regardless of pressure" I could have agreed!
Oil pressure (OP) is VERY SIGNIFICANT and in many valve trains it is the prime actuator in cam timing changes especially where vane drivers are used and clearances are very small to avoid noise and etc. In others it is the prime actuator for electrical phasers etc that control such things and in many other hydraulically controlled situations within the engine too
In fact OP is now much more significant than ever before in evolving engine designs.
Now, if we were talking about a Blue Flame 6 then, that's another story!
Personally I would have no hesitation in using an Approved and Listed 0w-20 synthetic oil in an engine designed for it (I did - in 1996 in a new V6 Explorer). After all in MY1941 the most common oil recommended was SAE20 for temperatures up to 90F! Lubricants have come a long way since then!
This has been a meaningful thread with many of us thinking we are correct and probably being incorrect - and vice versa!
The beauty of BITOG no doubt!