[jmohn]

While your noticing it probably is heat related your '83 should not ping on 91 octane (I've been driving (stop and go) in 90+ heat the past few days with 87 octane with no indication of any pinging). Do re-check the timing, you can even back it off a little to stop the pinging, the only thing that will suffer is a little performance. Check that the heat range on your spark plugs is correct also, if they're too hot the hot plug tip can cause pre-ignition. Someone already mentioned carbon deposits. Changing to different weight oil would have no effect on whether the engine pings or not. However, 30 weight, in my opinion, is way too light for your climate. Check the manual, I think 15-40 is the lightest recommended in your temperature, personally I use 20-50 (synthetic does give you a larger margin, but I still wouldn't go as light as 30).

Jerry M
'78 SC