Magnuson-Moss Act (The Magnuson–Moss Warranty Act (MMWA) (P.L. 93-637) (15 U.S.C. § 2301 et seq.). Enacted in 1975, the
Magnuson–Moss Warranty Act forbids tie-in sales provisions in warrantees. These provisions would require a purchaser of the warranted product to buy an item
or service from a particular company to use with the warranted product in order to be eligible to receive a remedy under the warranty.
Here are examples of prohibited tie-in sales provisions.
1) In order to keep your new Plenum Brand Vacuum Cleaner warranty in effect, you must use genuine Plenum Brand Filter Bags.
2)Failure to have scheduled maintenance performed, at your expense, by the Great American Maintenance Company, Inc., voids this warranty.
So you can use any suitable oil filter; CAI, battery, exhaust system etc. so long as that add on is not the warranteed part. You can also have routine maintenance performed at your expense at a qualified repair facililty without voiding the warranty (changing your own oil without being an ASE certified mechanic has been a problem for some in the past however, especially when the 5.2L were sludging up oil sumps and spinning main bearings).
While the manufacturer cannot use a tie-in sales provision,
the warranty need not cover use of replacement parts, repairs, or maintenance that is inappropriate for the product. The following is an example of a permissible provision that excludes coverage of such things.
While necessary maintenance or repairs on your AudioMundo Stereo System can be performed by any company, we recommend that you use only authorized AudioMundo dealers.
Improper or incorrectly performed maintenance or repair voids this warranty.
Although tie-in sales provisions generally are not allowed, a manufacturere can include such a provision in their warranty if they demonstrate to the satisfaction of the Federal Trade Commission (FTC) that their product will not work properly without a specified item or service.
Here is a handy interpretation of the MMWA:
MLM Law - Lawyer Grimes & Reese PLLC - Attorney Specializing in Multilevel Marketing - Understanding the Magnuson-Moss Warranty Act\
Bottomline: if the add-on is not the cause of the failure you may still have to lawyer-up, but eventually the dealer will have to eat it under warrantee.

Everyone about to fall asleep yet?