GMC Hummer H3 2008 Owner's Handbook Manual page 247

Service and Maintenance
NOTICE
Your vehicle was not designed
for fuels that contain methanol.
Methanol can corrode metal
parts in your fuel system and
can also damage plastic and
rubber parts. Damage caused
by the use of fuels that contain
methanol would not be
covered by your warranty.
. . . 246
The engine in your vehicle is
designed to accept unleaded
petrol only. The use of leaded
petrol could affect your vehicle's
performance as well as your vehi-
cle's warranty.
Use of the recommended fuel is an
important part of the proper main-
tenance of your vehicle.
You should use a high quality fuel
which meets the minimum octane
rating recommended.
Be sure the posted octane is at
least 91 (research method).
The use of petrol with an octane
rating lower than that recom-
mended may cause engine knock.
Continued use can cause engine
damage and void the warranty.
If you detect a heavy engine knock
even when using a petrol of the
recommended octane rating, have
the engine checked. Short periods
of engine knock during hill climb-
ing and acceleration are
acceptable.