Since there are a lot of discussions on fuel quality and additives, I thought I would report my experience. Last year I bought a 19 yr. old(now 20) MB CLK 430 with the 4.3 v8 with 98k miles on it. It seemed to run a little rough so I filled it with Costco Prem. After 3or 4 tanks, I noticed it was running smoother and quieter. I used no fuel additives. The car had questionable maintenance so I changed all fluids. As a side note, one of the lifters was ticking loudly until the engine was hot. I put in 1.5 cans of LiquiMoly Hydralic lifter conditioner. It took several hundred miles but it has quieted down to normal.
I didn't realize how rough it was until it was cleaned out.
I didn't realize how rough it was until it was cleaned out.