Are you sure it is a lifter? I once bought a vehicle for very cheap because
the person thought it had a noisy lifter thus would need engine work soon.
After listening to the engine run, I bought it. It was a leaky exhaust and
the sound of the engine was coming out of it.
Agreed, make sure of the diagnosis.
I don't think using Mystery Oil (the real stuff) will hurt it, so it
may be worth a try, but OTOH, if it is really bad, it probably won't
help (likely a bent pushrod).
Might be worth pulling the valve covers and doing a visual inspection
if Geekboy's suggestions don't pinpoint another area. You should be
able to see which valve lifter(s) are having problems that way (turn
over the engine by hand and watch/feel etc.)
Also, what engine is this?
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