When using an oiled air filter, after the initial install or servicing, run it for a week or so and hit some hi RPM's a couple of times and then clean your MAF elements. If an oil residue accumilates there it will have the MAF lie in a negative manner that effect mpg, shifting, idle and so on.
There is a GM TSB stating if the symptoms described above are found and traced to an after market oiled air filter, it is not to be covered under warranty.
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