Doug:
1) When the oil temps went up, so did the cylinder head temps and so down went the threshold of detonation: voila,....pinging.
2) Caveat Emptor on octane boosters. You were warned/informed,..use that stuff at your own risk.
3) California dropped MTBE from all pump gas in late 2002. Thats when it changed from 92 to the current 91.
If you want to truly help prevent pinging, do something worthwhile to reduce your operating temps. The very best and ONLY effective was is to add an additional front oil cooler. Its not cheap, but you get what you pay for. The thermostat is not adjustable.