So what's the consensus on heat and hydrodynamic friction when using a thicker oil in tight clearances? With reduced flow exiting the bearings and increased hydrodynamic friction from the increased viscosity, the oil's temperature rise through the bearing must be higher and bearing temps also higher.
I've noted spalling wear of the rod bearings from someone using a 20W-50 in an road racing engine with .0015-.0018" clearance. The mid-season oil change (after 5 events) showed >100 ppm of both Pb and Cu with a bunch of tin as well. The oil pan was removed, rod bearings pulled, and found spalling of the bearing surface. Oil sump temp would reach 280-290*F during the race with the bearing temps likely >350*F. Now, also note this was a '65 289ci V8 engine with stock replacement bearings. A modern engine with better alloys and aluminum block would likely take that heat better. The crank of that engine was still decent so it was just cleaned up and new bearings installed at the same clearance with a 5W-30 (Redline) used. The racing oil temp dropped down to ~260*F and they looked good as new after another 5 events.
I know Ford recommends Motorcraft 5W-50 for racing in the higher end models, but I don't know of anyone competitively racing those cars that actually uses a 5W-50 oil. The few I've had the liberty to check out were using 5W-30 or 0W-30. The drag racers are using 0W-20 or 0W-16.
No doubt that the oil temperature inside the bearing is going to heat up more with tighter bearing clearance, regardless of the oil viscosity. If the bearing clearance it too tight it could over heat even with lower oil viscosity. Another factor is how does the bearing clearance change as everything heats up? If the expansion coefficients of the materials used in the rod/crank/bearing are the same or very close then probably not a real factor, but who really knows.
Per the SubsTech link I gave in post #21 the oil shearing is higher with a smaller MOFT. A bearing with a low clearance will naturally run at a lower MOFT as compared to a bearing with a larger clearance (using the same oil) because the MOFT is a function of the clearance per the bearing Sommerfeld number. A thicker oil in the same bearing will run at a bit higher MOFT, which means the sear rate will be less, but from what I'm seeing it would still cause the oil temperature to increase a bit more even though it would be shearing the thicker oil at a lower shear rate.
There are two opposing factors going on at the same time. And of course, as the oil viscosity inside the bearing heats up more and more then the MOFT keeps going down and it kind of becomes a run-away situation. If it gets to the point of some level of metal-to-metal contact then things heat up even more. That's why keeping sump oil temperature in check in any race car with a good oil cooler is really key to the oil and engine surviving. A thinner oil may help reduce oil temperature, but it also decreases MOFT, so again there are two opposing factors going on, and which one will prevail to ensure the MOFT doesn't go to zero?