Pls help...6 cyl or 8 cylinder Luxury autos ?

My sister in California is thinking of buying a "used" Luxury car /
sedan  around 5 years old  like a Mercedes or Lexus.

She is confused as to  the  gas mileage  for 6 cylinder  and 8 cylinder
luxury cars  and the related  cost of gas.

I was told that for normal driving say 10,000 miles or so in a year,
the cost difference wasn't much on a weekly basis even with these high
gas prices.

Is the story any different with 6 cyl and 8 cyl SUVs ?

Pls educate us on this as we are NOT all knowledgeable

Thanks
Anna