I was reading an article about how required octane levels are dictated by the compression ratio on an engine (see here), based on the fact that the V6 6A12s in GRs and GPXs have a compression ratio of 10:1, you should use 100 RON fuel (I know, you already knew that ), thankfully the ECU handles our 95 octane stuff handily enough. Its interesting to note that the GS 4G93 engine is rated @ 9.5:1 so running it on standard unleaded is not a problem.
CJ