I also own a 2016 Silverado Z71 that had had an issue with increased oil level, fuel smell in the oil, decreased fuel mileage, and a heavy raw fuel smell on start up. As others have noted, if it doesn't throw a code, the problem doesn't exist. (IMO: This is the difference between a mechanic and a technician.) I have asked my dealer to pull a fuel dilution oil sample, but they still won't and tell me I need to get it done myself. They've stalled this issue to the point that the bumper to bumper warranty is now expired and I've gotten GM Customer Service involved.
So far, the result is the same. "We can't find anything because it's not giving us a code!"
My truck now has 34K on it and this issue has been present for about 20K and I've even been accused of adding extra oil.
I know it's a leaking injector or two. Rough, noisy start ups, rough idle cold or warm, and a drop of about 6 to 10 mpg over all.
If others suffer from this same problem, I'd like to hear from you, problem solved or not. It's past being old and into the realm of total stupidity. This should be documented to GM and addressed.