Interesting paper. Figure 2a tells me that bearing wear for viscosity below the current minimum of 2.6 mPa.s goes off the cliff with the big step change in the curve. Looks like main bearings aren't as sensitive to low HTHS like rod bearings."Additional work demonstrated that a similar step change in wear can be observed in piston ring and cam face wear ."
What I don't get however in Figures 2a and 2b is why does the bearing wear increase the other way as the HTHS and MOFT increases? More bearing wear on cold starts with thicker oil possibly? Figure 2a would indicate that xW-20 with HTHS of around 2.5~2.6 gives the least bearing wear.
SAE paper that Figures 2a and 2b references: http://papers.sae.org/892154/