I have a 2004 XJ VDP with 49k miles. I've had it now for 8 months and love it. I only use super unleaded, always. I've been noticing some light engine knocking under hard acceleration. No check engine light, no nothing. Spoke with my independent Jag mechanic and he said to only use Chevron 91. Made the switch and the knocking is completely gone. The engine feels/pulls stronger as well. It's a little more expensive than Shell, 76, etc but totally worth it.

 

If the Shell octane rating is 76 then it is no wonder you're getting engine knock, but I suspect it is higher as most will be in the 80s and 90s. Shell is normally a high quality fuel, but much can depend on the garage that stores it and how much they sell per month.

 

76 is a brand of gas. Octane was 76 in 1932. Now part of Chevron.
https://en.wikipedia.org/wiki/76_(gas_station)

 

I almost exclusively use Shell, 93 Octane. used Mobil but found I got better gas mileage with shell in my 2008 XJR. Never had any problems with any Premium gas I put in any of my 10 Jags over the years.

 

No modern Jaguar requires or can make use of anything higher than 91 AKI (95 RON) octane rating.

Octane rating has no connection to quantity or quality of additives.

All brand name gasoline retailers, as well as the Mom and Pop stores or supermarkets, all get their fuel from the same refineries. The only potential difference is with the additives. Difference in additives will take months or years to show up.

AAA did an excellent in depth report blowing all the common myths out of the water. The net difference from best to the worst gas was less than 1 mpg and it took 3000 miles to show up. No other difference.

If you suspect that you were being sold low octane fuel at the price of high octane, please register a compliant.