I've got an 01 c3. It's got a 6.0,awd,tow package and every single option possible.
The C3 has a different cam than the 6.0 in the 3/4 ton of the same year and has more hp/tq as well.
Basically it's identical to the escalade truck without the badge.
These engines are bulletproof. SteveSRT sold one of his work vans with this engine and it had half a million miles and still going strong for the new owner. Engine had nothing more than fluid changes.
So 125000 is nothing. Mine has roughly 120000 miles and runs stellar.
As far as oil goes I'm a pennzoil guy however this engine family isn't picky. I run 10000 mile intervals,ignoring the oil change light. If you want to follow the oil life monitor a conventional is more than adequate.
As far as brand is concerned it doesn't really matter. Oil is sold in tiers. Mobils top tier oil is basically equal to pennzoil's top tier oil and so on.
As Trav has already stated you can't go wrong with a dexos approved lubricant as well.
As far as quality goes my truck runs great,all the options still work great like the windows and so on.
Even the mirrors and heated seats are operational. Not bad for a 13 year old truck operated in sask winters for 6 months of the year.
Comparing it to my former hemi truck the c3 feels like it's got more bottom end torque. The numbers between them are very close,the hemi had 340hp/390tq. The c3 has 325hp/375tq from the factory however I bought a tuner and had the maps adjusted so the numbers are likely much closer.
Overall I am very satisfied. They are very heavy but well powered and almost nimble,and the awd is fantastic.
Enjoy your new ride. You'll tire of it long before it wears out.