i know a guy with an LBZ that ran B20. He didnt have problems with it and fills up with it whenever he can. I think he leaves like a 1/4tank of #2 and then fills up on B20.
I had a 96 Z71 as my first truck. When my tranny went 1-2 was sloppy but 2-3 would sometimes not even engage. It would just sit there and slip. Had 160k on mine. Smell the fluid. If you know what burnt tranny fluid smells like its a pretty distinct smell