down shift jerk

[hey1]

2001 x5 2001.I just noticed that after i drive for 30 km or so I noticed that when i brake and come to a complete stop the truck jerks a bit and brings the rpm down .I guess its downshifting from 2nd to 1st which is normal but not to feel the jerk.It happens either after I brake just before the truck stops or coasting in gear before stopping on its own.The tranny was rebuilt 3 yrs ago and the guys who did the job did not put bmw oil nor was it synthetic and added a fluid modifier.I assume thats what caused this problem.Today i was able to drain Just the fluid in the pan only and will have the fluid completely changed later.There is about 3L of old oil in the torque converter
I have not got the adaptations reset yet until I get all of the oil flushed out . What I dont get is after driving the truck why is the problem still there and or is it because the job is not completely finished yet .Will this flush correct my problem when all oil is new and having the tranny adaptations reset.Do i need to get it reset or will the truck adapt on its own?how long does it take to learn itself?
Other than that the truck drives perfect with no slipping or nothing
Thanks let me know what you think