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How this happens is a property of the base oils & additives.Using a thicker (higher viscosity) PETROLEUM oil (than the stock 80, 90, or 80W90) ending in 120 or 140 or 145, will take the transmission operation out of the design operating area, as far as gears & parts speeding up & slowing down during shifting due to oil friction, etc, thus gear changing up & down can be different, and may give problems.I recommend that you NOT use any additive if your transmission is filled with synthetic oil.

In some instances seals & the synthetic oils may not be perfectly compatible.

A straight weight oil, such as SAE 80, will thin much more, than a 80W90, as temperature rises. BMW's transmission designer/manufacturer, Getrag, specifically designed the transmission to use either straight 80, straight 90, or 80W90 multigrade, all in only GL5.

For such as a 80W145, as just one example of oils with a much higher top number; as your parts & oil rise in temperature, the oil thins less, & the specific lubrication qualities of friction inherent in thicker oils, is modified ... Just in case you were 'guessing' here ...no, the objective is NOT super high viscosity index.

While there can be other causes that a change to a thicker oil will not help, in this instance you CAN try the thicker oil, such as 85W140 or similar.

My present recommendation is that, if you want to, you may use a synthetic gear oil of good quality.

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