Toyota Corolla 92

Good day friends, I have a automatic Toyota Corolla 92, yesterday I began to slide the setback, the forward march works normally, a while ago (10 months) apply additive for the box (winng) for some small sliding forward and which were satisfactorily corrected, my question is, it will be that the oil loses bizccosity and does not apply the changes as it has to be or will have to repair it. That he is 4 years old to be repaired, but he has had very little use, sentences 90,000km, this is the first time he slides the setback and in such a surprise way.