Well, a few weeks back, I complained to Ford about a bad engine tick between 1000 and 2000 RPMs. The tick sounded like you were clicking the ignitor on your grill or hot water heater. Ford's fix was to put a mystery additive in the oil. The tick went away immediately, and Ford alleged the solution would be permanent. See here: https://www.allfordmustangs.com/forums/2011-mustang-gt-tech/250613-2011-mustang-5-0-engine-problem-fords-strange-fix.html
I drove the car the next 1500 miles or so, and the "ignitor" tick didn't come back (I thought I heard it once, but it was so faint that I wouldn't have let it bother me). However, the idle quality began to slowly deteriorate at idle. It always had a funny tick/chirp on the passenger side that Ford said was "normal," but now it sounds like it has a stereotypical "lifter tick" when warm at idle and just off idle. It sounds awful.
But wait, it gets better . . . .
When the additive was put in, I was told I could still change my oil at the next scheduled interval, and the ignitor tick would still be gone. So, I changed my oil at it's normal interval, and the noise immediately came back with a vengence!! So, I now have the awful ignitor sounding tick right off idle, and I now have what sounds like a lifter tick at idle (I realize the car doesn't have lifters, but that's what the tick sounds like).
But wait, it gets better . . .
When I brought the car back to the dealer (who has been outstanding in trying to help me with this), they contacted Ford. Ford wants the dealer to try and isolate the "lifter tick" to one bank or another, thinking it might be bad lash adjusters. As for the "ignitor tick," Ford wants the dealer to drain two quarts of oil out, and replace them with 75w-140 rear end lubricant! They claim the noise will go away with break in, and that adding two quarts of this thick lubricant will speed the process up. Of course, they can't say how long the car must be given to "break in." Note, I have about 5000 miles on it now.
So, the owner's manual says that the car needs 5w-20, but they want to replace 1/4 of the oil with something about as thick as tar.
This is unbelievable . .