In the summer I use 10w-30 in my '89 Sierra in the summer I use 10w-40
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The 2008 GMC Sierra-2500HD has a 6-speed automatic.
The 2006 GMC Sierra-2500HD has a 5-speed manual.
The 2014 GMC Sierra-2500HD has a 6-speed automatic.
The 2012 GMC Sierra-2500HD has a 6-speed automatic.
The 2008 GMC Sierra-3500HD has a 6-speed automatic.
The 2007 GMC Sierra-3500HD has a 6-speed automatic.
The 2013 GMC Sierra-2500HD has a 6-speed automatic.
The 2011 GMC Sierra-1500 has a 4-speed automatic.
The 2001 GMC Sierra-2500HD has a 5-speed manual.
The 2011 GMC Sierra-2500HD has a 6-speed automatic.