Gentlements,
I have several point to raise up.
1 - Resetting:-
Resetting is just tell your car that, this car had been services and everything back to normal setting (clear history). The actual problem is still not yet resolve.
Example: For 520i - You will feel a double jerk when you press the break at the traffic light or etc. This symptom is a norm for 520i as it is trigger by ABS and the only way to remove it is by resetting (logic). Normally the mechanic will not do it for you and quote with all the cinderella story, they would normally reset it when after changing the ATF oil only or some even after servicing. However, it would come back after a while when ? try to tab your break tripple time during your breaking...then you shall know.
2- ATF oil
The very first symptom which will trigger your attention is the mild vibration (Pls do not use the words jerk as it would not lead you to the actual problem) after you ignite your car. One of the common solution is to change the ATF oil, and i would only touch on this topic ATF oil.
If you change your ATF oil, i would strongly recommend to change the the gasket. During or after the mechanic remove the oil pan, please inspect the oil pan your self. You may see the amount of oil compilation and the wear and tear of the gasket. Normal practice, you should change the gasket but not the oil pan unless you identify mild crak and wear and tear symptom.
As for the oil pan, i would strongly recommend to use back the OEM part but not the Taiwan version. No regards how much credits given to the taiwan replica still cannot compare to the OEM.
FYI -As for the gasket, for those who are a mechanical engineer they will know this. There are method to close back the gasket and majority of the mechanic just simply close it together with the oil pan and cause damage to the gasket. You might not able to identify it so easily unless you go down inside to inspect it ur self. Just be caution and good luck.
Remember, you only need to use 6 bottle of ATF oil for 520i, why ? no regard how much they drain out the oil, there are still some percentage of oil inside. 7 bottle is consider completely dry out which is not possible at all.
Thanks for reading this lenghty reply hehe.....please take note, 520i is not quoted as sluggish or under power (determine by the car condition.), you just need to understand it especially on the timing and when the power delivers. If you are looking for those car which can deliver G force to you, then you better change you car as 520i cannot do it with the "D", "DS" and manual, it have to combine and also depend on situation. Any way, i still love my car as i would not be boring by only playing with one mode.