If its not pinging or burning the tips off the sparkplugs then your only worry is if you can really hear the pinging. Lug it up a hill if it doesn't ping use whatever suits you. Those motors are tough as nails and the factory settings on timing are conservative as can be. Personally I would use high test and jack the timing up ten degress, but then I like POWER!

Often the mileage will improve so much using high test and high timing that the economics are the same.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician