No matter what I guess there will be strict orders in the shop to ONLY use the SPECIFIED fluids! I still find it strange when you have known people who know when they need to add fluid to their own differentials when they start to whine and seem to drive them without issues for quite a long time this way.
Modern tolerances are probably a lot closer and that thicker fluid just didn’t cut it.
One thing is for sure. Some people guarantee job security you have to go around and fix all the problems caused by others. I see it in my line of work and everyone in any service business sees the same exact type of thing. Sometime it is the home repairer but sometimes it is a so-called professional as in this situation. I had one this week where over half my day was spent cleaning up the mess caused by someone else so that I could then fix it correctly. I mean things were just blatantly done WRONG and likely to cause more problems down the road. I see lots of problems caused by the guy they had used before me so this didn’t surprise me. The only reason he stays in business is because he charges 1/2 of my price or anyone else for that matter. You get what you pay for!