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Someone on a Jeep forum posted said that his Cherokee's manual said to use high octane for high temperatures and severe use. I think it was an OBD or OBDII model. I haven't heard of anyone else's manual saying that, so it has to be specific to that year. So under specific conditions apparently the old 4.0 does need higher octane.
Since I have overheating problems with my XJ, with intake air temp even reaching 130F, I've been switching between 93 and 87 octane. I think it does run smoother on 93.
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