I grenaded my slant 6
I don't know either, but in looking at how the lube oil flows in a slant, I can determine that:
rod ends 2 & 3 are oiled through #2 main
rod ends 4 & 5 are oiled through # 3 main
rod end 1 is oiled through #1 main
rod end 6 is oiled through #4 main
so rod ends: 2,3,4,5 are split oil flows
and rod end:1,6 are dedicated oil flows.
the rear main oil galley also feeds the oil line up through the block and head to the rocker arm shaft.
so there is definitely more oil flow demand towards the rear half of the crank.
Perhaps #6 is not a known bearing spinner, because of the dedicated oil supply.
So why is #5 and not #4 reported most often with a failed rod bearing, they may be both so close to failure that when the furthest one from the oil pump
(#5 vs #4) goes, the other is not far from failing also. Or it may be that #5 is actually the furthest rod bearing away from the pump (on a line that is shared and is also supplying a secondary lube to the rocker arms) so it fails catastrophically first.
Saying all of this without any empirical data. It would take a computerized 3D model to know with confidence, but for an on line argument I think this has some substance.
It is also important to note that the OP had a failure on #1 cylinder, and to me that rod failure appears to have initiated at the piston pin end, not the rod end. But that is from looking at one photograph of parts picked up from the road side. So there is a large opportunity for error on that failure analysis.
Meaning, the reasoning on why #5 rod bearings fails is a WAG, Wild *** Guess
and the reasoning on why the OP #1 rod failed is a SWAG, Super Wild *** Guess
