Each oil change (7k miles interval). I tried to be the smart arse in my last oil change (switched to Valvoline 5w30 synthetic with seal conditioner and no lucas additive etc) and the stinky smoking immediately came back with a vengeance! I reverted to the magic formula and all was fine in just a few miles of driving. It's amazing actually and I don't care for the long term effects to the engine, if any. We tend to treat these engines with kid gloves but they're just machines like any other. Lucas products has great reviews and no wonder it worked and my case is a great testament to that. I have not read anywhere where using lucas made the engine blow up or suddenly fly your car into the ocean. The only complaints I've read is that it did not work, but I think that would be in a case where the seals are completely deteriorated, or the product was not used per instructions. It's important to follow the instructions posted on the bottle. As for me I started using it soon after my car started emitting the stinky burn oil smell smoke.

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'05 E53 X5 4.4i, '97 E39 528, '07 E92 335i, '16 F86 X6M.