It seems your car's bearings really like/need thick oil. While I'm a proud member of the "Mo thickah, Mo bettah." crowd, I do find it odd that something like 0w-40 is so 'thin' as to cause excess bearing (lead) wear. I hope the car isn't sick.
Anyway here are some High Temp High Shear numbers for the syn oils I could find. The HTHS viscosity test is designed to model the oil's performance in the bearing. Higher numbers produce a thicker hydrodynamic oil wedge, all other things being equal.
PP
5-50 4.1 (Kinda wimpy for a 50)
15-50 5.2 (More like it! Is this available in your area?)
M1
0-40 3.7 (Not all that thin, although it does have a reputation for shearing with use.)
10-40HM 4.06
15-50 4.5
5-40 truck 3.9
Redline (You want HTHS, you've come to the right place!)
10-30 3.8
10-40 4.7
15-40 4.7
15-50 5.8
20-50 6.1
Castrol and Valvoline offer no HTHS values (that I could find) for their Syn stuff.
IMO, I'd look for something a little thicker than the PP 5-50. If M1 0-40 is causing problems, I'd aim for an HTHS in the high 4.x ~ 5+ range. (Just a wild arsed guess!)
The M1 15-50 actually seems a little thin by these standards, but it has lots of additives and should be more stable than the 0w40.
Try some Redline 10w40 (perhaps spiked with a qt. or two of xW-50) and post UOA results! 4.5K is a somewhat short oil change interval. Getting 6K should be easy for any of these oils, and will help justify the per-quart expense.