Would you consider sump oil temp to be not important compared with post cooler temps?
They are essential, but not for the viscosity decision. The sump temperature represents the collected oil from: oil squirt back flow, head back flow, bearing back flow and additional aggregates like Turbo, SC and so on. We can assume this to be the average temperature on the high end (not peak which differs locally). The conclusion out of that would be, this value is important for at least three properties: extraction of lighter fumes capability, thermal stress on the oil (degeneration rate) and a pre-cooling (mixed) heat extraction. The first two properties create a sort of bandwidth of necessary temperature, it should high enough to vaporize condensed or collected water (combustion!) and fuel but also cool enough not to oxidize/crack the oil too fast.
The later point is something which is especially something interesting when running an higher content of viscosity enhancer molecules, which are plenty in a 10W60 or 10W50. These polymeres crack with time and temperature. If the temperature is higher, they crack faster, if time is longer, they crack "faster" too. When they were cracked the viscosity drops as these molecules are responsible for the higher viscosity end, the 50 or 60 weight rating. This is the cause I would never ever use one of such oil formulations on an engine which see higher oil temps and longer usage time. This are racing oils, which are changed in very short intervals, like every race or so.
Base viscosity = function of bearing clearance, oil temp before bearings and some other stuff
Oil change interval = function of temperature household in the oil sump and oil path and other stuff
What you can't read in a datasheet of an oil is it's wear protection capacity. This is a mix out of the integrates and the base oil. A good oil design can be assumed of the ingredients and their concentration, but finally it has to be run and tested in the specific engine and application. My experience on that at an OEM engine company was even the oil companies don't know exactly how they should optimize their receipt or formulation of the oil. Every company has it's own procedure, different sources for ingredients and philosophy while the OEM's have the Norms and rules they demand for, like oil specification according ACEA and many others. Not every Norm or rule gives us the optimum for the application, even if it is done exactly for that purpose. Norms are just the minimum level which is demanded. E.g. Amsoil Signature Series 5W30 had for years no ACEA certification, but exceeded the specification for it. And it offers an amazing wear protection similar like the Ravenol VSW 0W30 does it, which full fills ACEA C3 and one of the hardest OEM oil specification of VW and have a GM recommendation.
Oil specification is one of the biggest discussion topics in engine development and testing. I don't know any single engine engineer who has all, the chemical, the physical and the necessary tribological knowhow to decide the best oil from a discussion of oil properties, I also don't know no single Oil company who made a direct hit for an specific engine without testing it. The only I know who are in both partly are guys who do scientific investigations on a single field of that much bigger topic oil-tribological system. So finally we all have to find it out the hard way, one take a longer road started with an oil which is far from optimum even if spec'd for the application and one take a shorter road, started from an almost optimum oil.
A lot of tuners still recommend, just to be on the safer side for themselves, irresponsible high oil viscosities. No one tells their customer that oil flow is the base number to start with, all tells them you need pressure. But oil pressure tells nothing about the oil flow. If there is an obstacle in the flow path, oil pressure will be high, but flow not. So is oil pressure the correct value to rate oil viscosity? No it isn't. Is oil viscosity an indicator for wear protection? No it isn't. What the heck does oil pressure help for? It is an indicator of the healthiness of the system. Once you choose an oil viscosity, oil pressure should always at the same level for same engine speed-load points. If it changes, worry about it! So it is a relative quantity we are interested in, so to say the quality of the difference to the start value (if it was already an healthy engine). Nothing more!
Of course oil viscosity and film thickness are correlating, means an higher viscosity gives an higher film thickness. But one should take in account that film thickness is a function of oil flow, relative speed of surfaces and oil properties like viscosity, temperature and so on. When at 9000 rpm the oil flow is lacking, the viscosity can't rescue it. How do one know when is the right oil flow achieved? Without a measurement of it we just can assume it. This is engine oil system depended, so no general rule then this. Oil flow is 1st priority, oil film thickness 2nd priority. So this means find the lowest viscosity oil giving you the right film thickness. Back again at what I mentioned above 😎.
