Hello Bruce,
no problems with deposits/sludging that I am aware of Bruce. M1 5w-50 has been a real success story for Mobil here in OZ and I have not heard a negative report - except for the price!
It was tried as a "fix" for the high oil consumption in alloy GM V8s sold here (M1 10w-30 factory fill) and it very worked well
GM fixed most of the engines under warranty so it became a non event
As Rick has indicated M1 5w-50 has shown little decrease in power and I can confirm that it was only M1R that knocked about 10hp off the V8Supercar engine used as a test vehicle for this purpose
You can purchase M1 15w-50 here too but it is usually by special commercial order or via the motorbike Industry
I had a little to do with the debvelopment of Castrol's R synthetic 10w-60 in the late 1970s early 1980s. It is a great product too in the correct application. It is a very popular oil here too. Its specs are HTHS 5.4cP, 4700cP @-25C, 170cSt @ 40C and 24.1cSt @ 100C. Pour point is -42C. In various formulations, this oil has been sold here for about 37years. In its first formulation it used castor as the ester - the smell was beautiful - just like Castrol R racing oils!!!
You said;
"Just for fun using the Lubrizol blend calculator I came up with a 0w/60 using PAO base oil with about 15% VII with a HTHS of 7.4
I am sure that some applications have a need for this types of lubricant. While we have a technical move to less viscous oils (SAE10w,20, 20w and 30 were popular oils in US cars for MY'41) some engine seem to respond very well to quite high viscosity after the start up phase is over
Many people forget that engines with hydraulic/electro actuators for valves etc and other phasers need a low viscosity at startup or serious technical and performance issues arise. This is why I mostly default to the manufacturer's recommendations
Perhaps you may comment, but this is why I see a long period of use for oils of the 0/5w-30, 0w-40 and 5w-40 viscosities too
Doug