Terry Coombs wrote, on Fri, 12 Dec 2014 17:57:34 -0600:

And I had to guess which spark plug to use, so I guessed that they
didn't mean R12YC but perhaps they had meant RC12YC?
about your spark plug , but i know my Harley runs an RN12YC . There are
charts online of what all the numbers and letters mean ...

Oh, we spent more than 45 minutes trying to find a RC12YC.
The auto parts guy looked in every cross reference he had.
We googled it on my iPad.
And we even called Generac from the store (which is what took
most of the 45 minutes).

Generac has no clue but they said it 'could' be a typo.
All we know is that the R12YC doesn't exist.

The one good thing that came of the multiple calls to Generac
support was that each person told us something different, which
allowed us to figure out that there is no such thing as the
"oil makeup tank" and that the "SAE340" was a typo.

We now have "revision 1" of the manual, which still says to use
the spark plug which doesn't exist though.

Anyway, I'll pull and compare spark plugs (not that this will
be an exact test but if it's way off, I should be able to see
that as long as it's not off internally).