Toyota Corolla 92

Good day friends, I have a 1992 Toyota Corolla automatic. Yesterday, it started slipping in reverse. The forward gear works normally. Some time ago (10 months ago), I added transmission fluid (Wing) for some minor forward slippage, which was successfully corrected. My question is, could it be that the transmission fluid is losing viscosity and not shifting properly, or will it need to be repaired? It was rebuilt about four years ago, but it has had very little use, barely 90,000 km. This is the first time it has slipped in reverse, and in such a sudden way.