My car knocks and carries on when it's been idiling for a while coz she's done 277,000km.

I went to a respected auto parts supplier and told him the prob and he suggested using 20w-60 oil because being thicker it would help quiet the knocks from the bottom end.

I asked a mechanic and he confirmed the suggestion so I put the 20-60 oil in and immediately the tappets/lifters started making noises.

I drove it for 2 days with the thick oil and got so sick of the tapping noises.
It didnt just make these noises at startup.
When ever the motor was lower than 1100rpm it was tapping even after a drive.

Changed back to 20w-50 and the tapping has decreased noticeably but is still there.

Have I done permanent damage? Is there anyway to stop the painful noise of the lifter/tappets?