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I also have 09 335i with almost 80k miles all stock with Procede 2.5 added recently. I already changed trans fluid and filter (you cannot change 100% only about 5 quarts), added Burger OCC, and had walnut blast done. Also, changed spark plugs, 1 coil and switched to RLI oil. I had RPM stumble at idle before walnut blast and I do have it still although rarely (it was rare even before the blast). Trans seems to be a little jerky then shifting into 3rd, and there is no change after changing the fluid, but it seems quite more touchy after adding Procede (maybe extra power making it jumpy?). Maybe I'll try Alpina flash to see how it will go. If I was you I would not waste money on additives as most likely you will not notice any difference, plus you do not know how it will affect your trans and you will have to spend a ton getting most of the fluid out thru multiple fluid changes.
At the moment the stumble is what worries me, and I hope that it will not be fuel injectors, but that is the only thing I can think off besides HPFP failing.
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