I used Amsoil 2000 for over 4 years. I ran it at 50:1 for 2 seasons and then 20:1 for almost 2. Never did we loose any parts due to lack of lubrication, We has a KTM 65, RM 80 (2), RM 85 (2), 250SX, 300MXC & TM125E (2). 9 bikes different styles of riding and alot of different fuel. I had a rod go in the 250SX, 300MXC and a RM80. All of these were on there 2nd season of harescrambles. All had 100 race hours plus on them.
Not that my rave is over. Lets talk about your rant. :)
That does stink what happened. I would call Amsoil and get someone that cares on the phone. They will say that it is not an oil issue but if you can prove it then they will back it up. I have personal experience with other products that they backed up without any issues (years back). I have heard alot of KX crank issues in the last 2 seasons regardless of oil. The YZ's are normally strong. If you talk to most Honest builders they will tell you that Dirt and Fatigue are the main causes of rod failure.
Goodluck
PS.. IMHO stay with more oil like 32:1 or Even 24:1 regardless of brand with these Lil screamers.