At-205 can help BUT if the stem seals are passed the point where they cant be saved... that's it. you give it 5 hours of engine operation with a fresh oil change and if you didn't see any difference you're out of luck.

it's worth a shot at least. but don't expect a miracle and if you get one you're blessed.

i had really bad smoke at high vacuum on a chrysler 4.7 magnum... used at-205 treatment four times, one on each oil change... never helped one bit with blue smoke at high vacuum and idle
sure helped with rear main seal.
in the end, all stem seals once replaced, zero blue smoke and no issues.

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2003 4.6, esotril blue, over 215,000KM. Every previous owner failed to keep up the vehicle maintenance. Restoration project.