What Octane Gas to Use

The following is taken from the manual for the 1985 Series III but should apply to other fuel injected XJ6's as well. The text was provide by Hartmut.

In the United States, Federal law requires that gasoline octane ratings be posted on the pumps. The octane rating shown is an average of Research (R) octane number and Motor (M) octane number. You should use unleaded gasoline with an octane rating of at least 87.

Using Unleaded gasoline with an octane rating lower than the stated above can cause persistent, heavy 'spark knock' ('spark knock' is a metallic rapping noise). If severe, this can lead to engine damage. If you detect heavy spark knock even when using gasoline of the recommended octane rating or if you hear steady spark knock while holding a steady speed on level roads, have your dealer correct the problem. Failure to take steps to stop such knocking is misuse of the vehicle for which Jaguar Cars is not responsible. However, now and then you may notice light spark knock for a short time while accelerating or driving up hills. Although this noise should not give cause for concern, it may be eliminated by the use of a fuel of a higher octane rating than that recommended.

The back cover of the manual for the 1983 Series III lists 91 octane, unleaded U.S. but, after years of paying for premium, I have switched to regular and it has made no difference to my 83. I have heard no knock under any circumstance. Try regular and see what you think.