Complex sentence: This leads to a change in transmission performance and so where unwanted performance/efficiency change has to be kept to a minimum a motor oil, or automatic transmission fluid, with a high viscosity index should be used.
Split into simpler sentences: This leads to a change in transmission performance and so where unwanted performance/efficiency change has to be kept to a minimum. Therefore, a motor oil, or automatic transmission fluid, with a high viscosity index should be used.