I replaced the gas cap on my 97 Mustang last year with one from O'Reilly Auto parts. Lately when I get gas some of the time the check engine light turns on. I loosen and tighten the cap and then the light goes out. Are there better aftermarket caps or can I lube the seal with silicone grease or vaseline?
Was it a Stant brand? I sold a great number of those at AAP and never had any come back.
Not sure about Mustangs in particular, but some vehicles seem to be overly sensitive to parts-store gas caps. I had a Chevy Trailblazer that would throw "small evap" codes constantly with caps from parts stores. Finally paid a few extra $$ for a replacement from the dealer, and never had another problem. YMMV
Silicone grease has worked for me.
The genuine Ford part is only $20 and still available...
I bought a cheapo one for my Mercury after the original cap split itself into 3 pieces, and it didn't come with the tether. 
Someday I'm going to get another OEM one.
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