10 quarts of 15w-40, although you will have to check your manual to see if
they recommend 30wt for cold weather.  I would recommend a fully synthetic
oil if you can get it and afford it, especially for cold starts.  Rotella is
a very good oil, and they make a synthetic Rotella as well.  I'm ashamed to
say it, but I use Amsoil, but only because I can't fine a fully synthetic
15w-40 locally and Amsoil is able to ship it to me without hassle in 1-2
days.  I don't sell the stuff and I'm not into their MLM, so after I got
that message across to them, they were more than happy to leave me alone and
just sell me the oil.  Oil changes are as easy on the Duramax as on any
gasser.  I installed a drain valve in place of the oil pan drain plug so now
it's even easier.  I also recommend a second fuel filter, because the LLY
injectors are picky about the quality of the fuel (a 2 micron filter would
be best) and they are expensive to replace.

You can reset your oil life meter by turning the key to ON but not starting
the motor, then depressing the gas pedal fully 3 times within 5 seconds,
wait for the reset message then switch the key to OFF.  That should do it -
enjoy your truck.

Cheers - Jonathan