What viscosity oil are you using?

__________________
1999 C280

I recently changed the oil to 15W-40W. In the beginning (a 1 1/2 yrs ago) the problem seemed to improve with a fresh oil change (regardless of viscosity), but its not the case anymore. Recently, I have tried thin oil for a week and then tried a heavier weight a week later but didn't notice a difference.

Thanks for the advice. I'll try replacing the lifters.