Engine oil changes nor brake pads do not have any effect on transmission. If battery was disconnected, engine control would be effected and that can cause changes to the idling speed which would effect the characteristics of transmission control.
I am not sure if you mean the slipping is when idling in gear vehicle rolls backwards on an incline.
This can occur depending on
1. Engine idling speed too low.
2. Transmission fluid level.
3. Gradient of slope.
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Tuesday, May 28th, 2013 AT 6:07 PM