90 Bonneville V6 3.8L Problems

I have a problem with my 90 Bonneville. You can read some of the symptoms from my previous post. I have 2 questions? What are the symptoms of a stuck valve? What are the symptoms of bad piston rings? I checked my oil today looking for signs of water. There was no milky look but the oil was real thin and smeeled strong of gas. How can gas get mixed with the oil? Any help would be appreciated.

Sparky

P.S. Sorry, it was three questions. The third is the most important. How can gas be mixed with the oil in normal use?