I bought a used 2009 GMC Sierra last month from a local Ford dealer. As part of their incoming used car inspection about a month before I bought it, they changed the oil and filter and did some other minor work. I was able to get copies of the shop orders, and noticed that they had put in 5W20 oil, instead of the GM-spec'd 5W30 oil. This dealer has a quick-lube section, so they just used their bulk Motorcraft synthetic blend oil. When I looked underneath the truck, I saw a blue oil filter, so I assumed it was a Delco. Today I changed the oil, and it was pretty dark for only 850 miles. The blue filter was a STP filter. I put in Castrol Magnatec Full Synthetic 5w30 and a Delco PF48E. The STP filter had a 9/17 date on it, As I looked into it, I noticed a pea-sized diameter glob of goop on the bypass valve tower, or whatever it's called, that was only about 1/16" thick when I scraped it with a flat blade screwdriver. I don't have a filter cutter, so I couldn't look at the filter innards. I have contacted the previous owner, who used the truck for commuting mostly, and he was good about getting regular oil changes at a shop whose owner I know personally. They usually use Chevron / Havoline products. Did the 5w20 loosen up something? This truck has AFM, but I just put a Range Technology AFM delete on it. I will watch the oil consumption, because before this, I had to add some oil, which was some Dollar General conventional SN-rated 10W30 that I had for my lawnmower. I also put some RMI-25 coolant treatment into the cooling system, since it needed to be topped off. The previous owner had the cooling system flushed / refilled about 2 years ago.