Well what I've seen with aftermarket fluid is not so much metal to metal breakdowns, but leaky seals, hardened seals, hardened o-rings, which then either lets foreign stuff in or lets oil out, or both in some cases. The ones that stick out are the front propeller shaft seals on some of the L series tractors. The shaft runs from the back of the transmission through the bellhousing. In the bellhousing there's one or two seals, a pair of rubber lined seal collars and a support bearing. Seen plenty of them where the collars rubber linings will swell, the collars will move on the shaft, the seals won't seal anymore, then oil leaks out of the weep hole on the bottom. The fix? Split tractor replace collars, seals, usually the bearing. About $1500. I asked what causes this and other issues that really don't have an explanation, and a japanese engineer told a translator that not using the proper fluid and/or lack of maintenance (changing fluids per the WSM) are the #1 causes. One can take that however they please. Matters not to me, I could use the work.