Timeing chain question on 90' 300SE

[80300dbenz]

I have a 1990 300SE, runs great and in pretty good condition for 180,000 miles. I was wondering do these cars have to have thier timeing chains replaced after a some particular point? Another thing is that I put regular unleaded instead of premium, and when im on the feeder getting on the freeway I can hear a pinging sound is that because of the UNLEADED fuel?

THANKS.