I am curious about how many people use Regular gas in their X300, and if they do how many miles have they put on the car since using it? My recently purchased 1995 VDP currently has 121k miles, and since owning it I have only put 500 miles or so on it using Premium gas (never Regular).

XJRengineer ?

The terminology and octane grades are a little different between England and the USA

The knock sensors should retard the ignition timing as it bumps on pre - ignition issues

Lower from higher octane aviation fuel going from 115 down to 87 auto gas required by law mechanically retarding the magnetos timing on aircraft

WW 2 aircraft used 145 octane

The car is capable of running on fuel more or less across the entire spectrum of available pump fuel.
I have been able to persuade myself that my XJR runs slightly more smoothly on premium grade, and with some advanced man maths, have persuaded myself that I enjoy 1-2 mpg better economy, thereby making it the only sensible thing to do!
Other strategies are available, and no harm should result

my cat has 375 000km on her. i believe that she only got there by being loved and deserves to be be pampered so only the best.
only premium 98 here. help her run nice and clean.
i know i feel better running on steak than i do running on mcdonalds and kfc.

I run regular gas. I've tried premium and found no difference.