Wording is everything, isn't it? I took this question and answer from the
site link you provided.
Question: Could using a 5W-30, 10W-30, 0W-30 or even a 10W-40 or 20W-50, oil
in my vehicle which specifies a 5W-20 oil void my new car warranty?
Answer: Absolutely not. Vehicle manufacturers only recommend using motor
oils meeting certain viscosity grades and American Petroleum Institute
service requirements. Whether a motor oil is a 5W-20, 5W-30, 10W-30, 0W-30,
10W-40 or 20W-50 (for racing and high performance applications in, for
example, a Cobra R Mustang) or even a synthetic vs. a petroleum based oil
will not affect warranty coverage. The manufacturer is required by Federal
Law to cover all equipment failures it would normally cover as long as the
oil meets API service requirements and specifications and was not the cause
of failure. In addition, the Federally mandated Magnuson - Moss Act states
that a manufacturer may not require a specific brand or type of aftermarket
product unless it is provided free of charge. If your dealership continues
to tell you that you must use 5W-20 motor oil and or/ a specific brand of
5W-20 motor oil, then ask them to put it in writing. Their position is
inaccurate, and, in fact violates existing law.*
Was I the only one to notice that above is stated,
The manufacturer is required by Federal Law to cover all equipment failures
it would normally cover as long as the oil meets API service requirements
and specifications and was not the cause of failure.
Key words here, "and was not the cause of failure".
So as I read this the manufacturer can indeed refuse to repair an engine
that was not running the specified oil even though the oil met "API service
requirements and specifications". If the oil was the cause of the failure
you are left holding the bag.
That said however the question was asked in such a way as to confuse the
issue. True, if you use a different oil than the manufacturer asks you to
use they still have to warrant the AC system, or paint, or electrical
switches but not an engine failure caused by an non specified oil.