Additives

hello, Just wondering if anyone has have ever used oil/fuel/... additives. If so, which ones, and did you noitce a difference? Are they worth time and money?

just another question... :) I want to try a thicker oil because my car smokes abit, but the car recommends 5w30, would using thicker oil hurt anything? I usually use 10w30 in my other cars, and I live in wisconsin, I know that 10w30 has a widder temp range, but I dont know anything about other oils...help plz!