Acid in the oil can cause the damages you see like on the rear main bearing, so can dirt #1 main bearing gets oiled first, #5 gets oiled last Have you looked at any of the rod bearings yet? If not look at them also, the main bearings get oil first and many times they will eat the dirt, if it is bad enough the metal and dirt from the mains will be washed into the rods also Let us know what you find BTW, how long has it been since the motor was built and ran for the first time years or months, not miles